Of cacti and humans – are certain life forms inevitable?

The search for alien life has been going on for almost 60 years, but so far, no one in the galaxy has returned our call. Why?  There are three commonly cited reasons:

1. Earth-like planets are very rare. Such planets have to be the right size, in the “Goldilocks zone” of their solar system (not too hot, not too cold, with abundant liquid water on the surface), the right distance from the chaotic center of the galaxy, have a metallic core, a relatively thin, mobile crust, and a protective magnetic field.  Nevertheless, with at least 100 billion stars in our galaxy there should be a few such planets around.  

2. The likelihood that alien technological civilizations occurring at the same time as ours is small. Planetary systems are of different ages within our galaxy.  Neither a paleolithic culture nor a long dead alien civilization, let alone a primeval world inhabited only by bacteria, is going to send or receive any radio signals.  In the 3.5 billion years life has been on our planet, we ourselves have only been capable of such communication for about 100 years.  Finding Paleolithic cultures (like the fictional Na'vi in Avatar) or the ruins of an ancient culture would be almost as much fun, but it's going to take more work than looking for radio signals.

3.  Technological humans evolved on Earth as a result of a series of lucky accidents that are unlikely to ever occur again on another planet.  John Gribbin, for example, in the September 2018 issue of Scientific American, contends that “Perhaps the most unlikely of all was the development of our technological species from those first sparks of life – a feat that is probably unique.”  This particular argument is one, however, that I believe is flawed and overly pessimistic. 

The question of alien life is analogous to asking “are there cactus-like plants in Africa?” That may seem like a stretch, but bear with me. In any case, it’s a good excuse to talk about aliens in a blog site devoted to plants! It is also a valuable opportunity to talk about the process of evolution.

It is true that Homo sapiens exists today as the result of a long line of very specific events.  My argument, however, is that similar results can arise in alternate timelines (or other planets) involving different lineages of organisms. This is because of the power of natural selection to create, often repeatedly, distinctive life forms that play particular roles (i.e. fill particular niches) in the ecosystem. The often startling evidence of this power is the phenomenon of convergent evolution - the evolution of similar organisms from unrelated ancestors. In other words, life on Earth-like planets will progress in predictable ways, becoming more diverse and complex, and result in a similar mix of life forms. 

So, are there cactus-like plants in Africa?  We have the advantage of being able to go to Africa and have a look - something we can't yet do on other planets.  The cactus family evolved in the New World and did not spread into Africa until very recent times.  However, two unrelated families of plants contain numerous remarkably cactus-like plants: the Asclepiadaceae (Milkweed Family) and the Euphorbiaceae (family of spurges, poinsettias and rubber trees).  The remarkable similarity among these plants is a spectacular example of convergent evolution. I might be so bold as to say that, given the existence of a variety of angiosperms to start with, the evolution of cacti and cactus-like plants was inevitable, because of their special ability to survive in desert environments.  Further, if all cactus-like plants were to become extinct for some reason, they would probably re-evolve from one plant family or another.  

The evolution of cactus-like plants is driven by an ecological opportunity - the existence of abundant sunlight in an area where the lack of water prevents most plants from growing. Evolving from forest shrubs, the ancestors of cacti gradually honed their anatomy and physiology through natural selection, becoming adapted to survive in the deserts. The distinctive leafless, swollen, water-filled stems of cacti are what we call stem succulents, and we can say that they occupy a very distinctive niche in the desert environment of the New World.  

Underutilized light, incidentally, was the ecological opportunity that drove aquatic algae to adapt to life in the terrestrial environment, becoming modern plants. Before that even, vast untapped sunlight is what drove the evolution of photosynthesis itself among ancient bacteria. Nature, via natural selection, does not let any resource go to waste indefinitely.

Stem succulents in the Cactaceae, Euphorbiaceae, and Asclepiadaceae have remarkably similar body forms due to convergent evolution.  Pictured are Mammillaria dioica from California (left), Euphorbia stellospina (center) from South Africa, and Larryleachia cactiformis (right), also from South Africa.

I could give dozens of other examples of convergent evolution in plants, including carnivorous pitcher plants, hummingbird pollinated flowers, and even simple categories such as "tree," "vine," or "geophyte," each of which has evolved many times in different plant families.  My argument will be that what is true for cactus and cactus-like plants on different continents could be true for humans and human-like species on different planets.

These remarkably dolphin-like animals are all reptiles

that existed in the Mesozoic era. Illustration posted on

Wikipedia, Creative Commons license.

Has convergent evolution occurred also in animals? Yes! Examples abound, including famously the repeated evolution of porpoise-like animals.   Among the Mesozoic reptiles known as Ichthyosaurs, many porpoise-like species, whose ancestors first lived on land, were hunting fish in the sea.  They themselves were repeating the body form of earlier bony fish and sharks.  True porpoises and whales were later iterations from land-dwelling mammals, but of course with more advanced mammalian anatomy and physiology. Sea lions, manatees, and penguins independently adopted the same basic body form that allows for efficient movement underwater.  Surely life on other planets will include one or more such fish-shaped animals.

Convergent evolution has resulted in numerous parallel life

forms between the marsupials of Australia and the placental

mammals of Africa and the northern continents.  Image

posted online at BiologyWriter.

Perhaps the most famous example of convergent evolution among mammals is between marsupials in Australia and placentals in the rest of the world. Marsupials entered Australia early on, while placentals either did not, or for some reason died out.  So as placentals adapted to the varied habitats and food resources in Africa and the northern continents, marsupials adapted to similar habitats and food sources in Australia.  So in Australia there are marsupial wolves, mice, cats, moles, anteaters, and grazers.  There are even arboreal marsupials called cuscuses that are like  primitive primates.  It seems then that these distinctive life forms appear inevitably wherever there are similar habitats and food sources.  In other words, when there is an ecological opportunity (an “empty niche,” if you like), organisms will adapt to exploit it.  

So if porpoise-like, wolf-like, mouse-like, and other animal life forms are evolve inevitably in response to particular ecological opportunities, why not humans? What does it take to make a human?  

To begin with, humans are mammals, rather than reptiles or birds.  Live birth, maternal feeding through lactation, and the accompanying parental care seem to foster the ability to learn or pass information among individuals, an important component of human behavior and intelligence. The ancestors of primates were probably small, shrew-like mammals that had co-existed with dinosaurs for millions of years, primarily by staying out of their way, coming out at night, or by scampering up into trees.    

Figs arose about 50 million years ago, during the early
diversification of primates, and are a major source of food
for birds, primates, and other animals in tropical forests
around the world..
Photo: Bernard DUPONT, Flickr CC BY-SA 2.0

Primates are traditionally thought to have appeared around 55 million years ago, when they began showing up in the fossil record.  A radically new theory, however, places their origin at about 185 million years ago, with other estimates inbetween. Either way, their diversification in the fossil record coincides with the development of tropical rain forests increasingly dominated by angiosperms, particularly those producing tasty fruits.  Tropical figs (of which there are some 850 species occurring throughout the tropics) first appear in the fossil record about 50 million years ago and have been a major food source for tree-dwelling primates, as well as birds and other arboreal animals ever since.  Angiosperm trees also have broad crowns with intermingling limbs, and some angiosperms evolved also into lianas that further lace the trees together.  This created a 3-dimensional, food-filled jungle gym perfect for the evolution of primates - an ecological opportunity that would almost certainly be exploited in any timeline or planet. 

Primates have flexible shoulder joints allowing for rotatable 
limbs, as well as grasping hands and stereoscopic, 
color vision. Photo posted by Glen Tarr on Quora.com.

Humans could not have evolved without some key anatomical innovations that arose during our ancestors' arboreal phase.  Yes, our ancestors had to have been monkey-like creatures, not dogs or gazelles or dolphins or octopi, and here’s why.  The hallmark feature of humanity is our ability to fashion weapons, tools, clothing, musical instruments, and ultimately space ships out of sticks, stones, and other natural materials.

To do all of that we need grasping hands with opposable thumbs, eyes forward on relatively flat faces for stereoscopic vision, and something that doesn’t get as much press coverage: rotatable limbs -  arms that can rotate around the shoulder joint and be lifted above the head. Imagine a dog or a gazelle walking up to a pitcher’s mound, winding up, and throwing a fastball across home plate.  They can’t do it because of the limited flexibility of their shoulder joints. Rotatable limbs evolved in primates, along with grasping hands, as a means of swinging from branch to branch in a tropical forest, as well as for reaching edible fruits and other food items in those trees. Such limbs were essential for the earliest stages of weapon technology - throwing rocks and wielding sticks.

The second crucial phase of human evolution, was, ironically, coming back out of the forest, but now equipped with grasping hands, rotatable limbs, and a higher level of intelligence, communication, and social interaction that also progressed in the trees.  Anthropologists still debate the ecological stimulus for the human debut, but it appears that a changing climate in Africa led to expansion of the savannas and greater variation of available food sources.  The evolving intelligence of our ancestors enabled them to recognize patterns and predict where to find food in this sparse habitat, as well as to devise cooperative strategies and weapons for hunting and defending themselves.  In the process, they became more upright in posture, perhaps to be able to see over the tall grass, but also to more comfortably carry and use weapons.  

Incidentally, when I was teaching introductory biology I did an exercise with my students critiquing fictional aliens.  It is highly likely that technological, human-like aliens would have had to go through a similar evolutionary sequence as us, and therefore would look boringly like us.  They too would first have to have lived in tropical trees and then have come down from those trees to have the necessary hand and shoulder anatomy for manipulating weapons.  So which are evolutionarily more logical - Hutts or Vulcans? Try to imagine how each would have evolved their human-like characteristics in some other way.

As intelligent as octopi and dolphins might be, they could progress no further in the aquatic environment.  Though octopi have some ability to manipulate objects with their tentacles, dolphins have none. Aside from that, have you  ever tried to throw a rock underwater?  How about lighting a fire or smelting iron for weaponry or tools?  Forget also Lady Proxima, the giant, apparently amphibious, worm-like creature portrayed in the most recent Star Wars Film, Solo - how could such a creature evolve into an intelligent being?

Was the evolution of human-like creatures  inevitable and repeatable?  All we can say is that it was ultimately  successful (perhaps too successful given our precarious environmental situation today!) and that under similar circumstances on another planet it would probably happen again.  If tropical forests filled with edible fruits evolve, creatures will adapt to arboreal life.  Climate change will inevitably happen, and primate-like creatures will very likely emerge from the forests and survive by their wits and their weapons.  

Was the extinction of the (non-avian) dinosaurs 65 million years ago a lucky accident necessary for the evolution  of primates and humans?  The question applies to mass extinction events in general, which clearly can significantly alter the course of evolutionary history (a theme of "A New History of Life," by Peter Ward and Joe Kirschvink).  The survivors of such events, however, will typically re-diversify into a range of ecological niches similar to what was present before. This is what convergent evolution shows us. Mammals, in fact,  took over many of the niches left vacant by the dinosaurs.  The actors change, but the roles they play are largely the same. 

As an analogy, think of a village of humans wiped out by some natural disaster, except for one surviving family.  That family first would have to do everything for themselves -  grow food, weave cloth, make candles, etc.  But as their numbers rebounded, they would begin to specialize for different economic roles, eventually specializing into the varied professions that had existed in the previous population. 

As for the dinosaurs, it might have been more difficult for humans to come out of the forest if T. rex had been still stomping around.  However, when considering alternate timelines, or other Earth-like planets, the dynamics between reptiles, mammals, and angiosperm forests could work out in a variety of ways.  Dinosaurs might have died out for other reasons, or if not, been no more of a threat than the large cats encountered by early humans on the savannas. See the interesting BBC post on this topic.

What if humans themselves had gone extinct for some reason? There is a theory that humanity  barely survived an incident that took place some 70,000 years ago.   Some calamity, possibly a super volcanic eruption, is said to have reduced the population of modern humans down to a few thousand.  This is based on DNA evidence suggesting that all existing humans today descended from a single small population.  Without getting into the debate of whether that actually happened, let’s suppose it did, and let's suppose our direct ancestors in Africa had been completely wiped out.  Again, depending on what survived, we might have re-evolved via a different route.  

That particular event apparently didn’t affect Neanderthals, at least not to the same degree, for our rebounding ancestors met them some 20,000 years later as they moved from Africa into the Middle East.  Neanderthals were already quite human, with hunting technology, clothing, and possibly religious beliefs.  They might have picked up the torch if our species in Africa had been wiped out. (Maybe the Klingons were the Neanderthals of another planet!).  Even if Neanderthals had been wiped out too, other surviving primates might have started the process of humanization anew. 

There are other unlikely events cited by the alien civilization nay-sayers, including the origin of life itself, formation of the first eukaryotic cells, and other important steps.  Was the evolution of multicellular animals and the progression from fishes to amphibians to reptiles and mammals inevitable? Would it likely occur the same way on another Earth-like planet? It would take a book to discuss all of these questions, though they could be analyzed in the same evolutionary framework I've used here, with examples of convergent evolution. I think the answers are generally yes.

Of course, all that I have said here is speculative, but my point is that when considering the likelihood of human-like life on other Earth-like planets we must keep in mind the power of natural selection to adapt organisms to new or recurring ecological opportunities.  Convergent evolution, in particular, strongly suggests that particular life forms, or “niches,” will be filled by organisms of different ancestry in different parts of the world (or galaxy), and refilled if emptied by some disaster.  Humans are not the result of a series of lucky accidents, but the product of ordinary evolutionary processes. As cacti are inevitable because of their remarkable ability to survive in a particular environment, so humans might be inevitable because of their remarkable ability to survive and dominate in harsh or unpredictable environments.

The odd seed baskets of carrots

Flowers of the carrot, Daucus carota, are borne in a flat-topped
inflorescence called a compound umbel.  Ignore the
foliage in this photo, as it belongs to a neighboring potato plant.

Most people don't notice the elegant inflorescences of the carrot plant (Daucus carota).  If you do, it usually means you've waited too long to harvest the edible, orange taproots.  If you have seen them, you might have noted the resemblance to Queen Anne's Lace, which is in fact a wild relative of the cultivated carrot. 

The individual white flowers are borne in small, flat-topped clusters called umbels, for their resemblance to little umbrellas.  The umbels, moreover, are grouped into a more compound structure, creating a large, flat-topped display for the tiny insects that will feed on the flowers and disperse their pollen.  The carrot family, Apiaceae, used to be called the Umbelliferae, after this characteristic inflorescence structure.  Celery, coriander, celantro, parsley, dill, fennel, and a host of other plants useful for nutrition and seasoning, belong also to this family.

Even fewer people have noticed the strange contortions these inflorescences undergo during their development and the maturation of their seeds.  As the young flowers begin to form, they are hidden and protected within the in-turned inflorescence branches and a series of spiky bracts.



As the flowers mature, the branches of the inflorescence expand and bend outwards to form the compound, flat-topped blooming structure.

The surprise comes as the flowers wither and their ovaries mature into the tiny, one-seeded fruits, that we superficially take for bare seeds.  The fruits of the  carrot and other members of the Apiaceae (sunk by some into the Araliaceae) are technically schizocarps, as they consist of two single-seeded units that split apart as they mature.  The fruit wall is thin, and dries into a hard outer layer on the seed, and so is unnoticed. 

As this ripening process proceeds, the branches of the inflorescence bend inwards again, bringing the fruits inside of  what is now a basket-like structure.  It is likely that this phenomenon is an adaptation for protecting the fruits from herbivores as they ripen, but I've not been able to find any literature to verify this.  The basket may also serve as a giant salt-shaker like structure that sways back and forth in the wind, helping fling the seeds away from the parent plant.  A similar structure has been noticed in the related genus, Conopodium, and is likely to be found in other members of the family.

As the young flowers develop, they are protected within the in-rolled
branches of the inflorescence.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.





.
.

As the ovaries of the flowers develop into the dry fruits known as schizocarps,
they are drawn inside of a basket-like structure, by the inward curving
 inflorescence branches.

.
.
.
.
.

.
..
.

Are palms giant herbs?

The largest inflorescence in the world 
is that of a palm, Corypha
umbraculifera, which like the well-
known Century Plants in the New
World, dies after its massive
flowering.  The inflorescence
is said to contain approximately 
24 million flowers. 
Photo courtesy Scott Zona.

The  Palm Family (Arecaceae) includes some of the largest monocots in the world, and one could argue, the largest perennial herbs.

To call a massive palm tree an herb may seem like a strange statement, since it has a sturdy upright stem, and may live for 100 years or more. Palm trunks may be a meter or more in thickness (Roystonea or Jubaea), and they hold the records for the largest inflorescences (Corypha umbraculifera), the largest seeds (Lodoicea maldivica) and the largest leaves (Raphia regalis) in the plant kingdom.

The largest seed in the world, weighing
up to 55 pounds, is that of Lodoicea maldivica,
from the Seychelles Islands.  The large
seeds are thought to be an adaptation
for survival of seedlings in a thick forest
 with nutrient- poor soil. Posted on
Wikipedia, Creative Commons license.

Members of the genus Raphia in Africa have the largest leaves of any plant. Pictured is R. australis, which is truly huge,
but a camera-shy relative, R. regalis, has the largest leaves, measured at over 25 meters in length.
Photo posted on Wikipedia, Creative Commons  License.

So is a palm a herb? The traditional definition of a herbaceous plant (or simply herb, in a botanical rather than culinary sense) is that it lacks permanent, above-ground woody stems, though they may have woody underground parts. Tulips and dahlias are examples of perennial herbs, while pansies and marigolds are examples of annual herbs. The alternate category is woody perennials, which include trees, shrubs and lianas.  There are, in fact, some dwarf palms that do not produce upright stems.  They would clearly be perennial herbs.  But what about larger palms?

The  vegetation of herbaceous plants is produced entirely through primary growth, in which all tissues arise from the apical meristems, or buds, at the tips of the stems. In contrast, woody plants exhibit secondary growth both above and below ground.  It is important to note that wood is the production of concentric layers of secondary xylem.

Are bamboos, perennial herbs or trees? Photo by Alain Van den Hende,
posted on Wikipedia, Creative Commons License.

Tropical plants, and tropical monocots in particular, severely strain the distinction between those two categories. First of all, no monocot, even a palm "tree," has true woody tissues. Their stems, no matter how thick or dense, are produced entirely through primary growth, and are strengthened by dense masses of fibers, rather than by layers of secondary xylem.  For that reason alone, all monocots could be considered herbaceous.

Many botanists would consider that too picky, and would use the term "woody" in a broader sense to refer to the dense wood-like tissues of palms.  And there are a few monocots, such as the dragon trees, giant aloes and some dracaenas, that have a specialized form of secondary growth, but such growth adds only layers of fibers and vascular bundles, not layers of secondary xylem.

Even if we accept that palms and other giant monocots are trees, there are still many gray areas where one is not quite sure where herbaceous perennials end and trees begin, and so there is value in pointing out the distinction between the very different ways that monocots and dicots form tree-like growth forms (see The invention and reinvention of trees).


Monocots abandoned the ability to form true wood as their ancestors adapted to a growth form based on rhizomes, with leaves that elongate from the base, and short-lived upright reproductive shoots (see How the grass leaf got its stripes).  Leaves of monocots, which can  be relatively large, are heavily dependent on bundles of fibers for support against both gravity and wind, as well as sometimes for protection against herbivores.  As they spread to a wide variety of habitats, some monocots got larger and developed upright stems with increased density of supporting fibers.  Important commercial fibers come from a variety of monocots, including Manila hemp (from a type of banana), sisal (from a species of Agave), and New Zealand hemp (from Phormium).  Fiber can also be teased our of bamboo stems and the leaves, stems, and fruits of many palms. 

Tropical monocots tend to be evergreen, another way they differ from temperate herbs.  Banana plants, which are tree-like, but clearly herbaceous, remain above ground for several years.  Others, such as agaves, aloes, and birds-of-paradise have permanent tufts or rosettes of above-ground foliage, typically arising from underground rhizomes.  No one would confuse such plants with woody shrubs, and these must be considered  perennial herbs.   Other monocots, including many grasses (e.g. canes) have upright stems that are reinforced with fibers and may last for several years. Bamboos are giant grasses with sturdy upright stems that live for many years (see The grasses that would be trees).   Should they be called herbs or woody plants?  Neither, actually.

The whole point of this long diatribe is to once again to point out how different monocots are from other vascular plants.  Their growth forms cannot be classified in the same terms as dicots.  They have mimicked the forms of many other kinds of plants (e.g. palms vs cycads), but with very different patterns of growth and tissues. Some of the elaborate classifications of the past (try googling: "plant growth forms") included special categories for palms and bamboos, but many did not.  In my opinion, the term "woody" should not be used for any monocot.  We can substitute the word "fibrous," which will be much more accurate and informative.  Many tropical and xeric monocots can be referred to as evergreen perennial herbs.  That would cover agaves, aloes, yuccas, and birds-of-paradise, as well as smaller palms.  Tree-like monocots, such as coconut palms, bamboos, screw-pines, Joshua trees, and dragon trees, might be called "fibrous arborescent perennials."  

Mosses of Central Florida 52. Fontinalis sullivantii

Fontinalis sullivantii Lindberg (Fontinalaceae) is a straggling moss often found in water, but also on soil or tree bases in moist areas.  Leaves are spread primarily on two sides of the stem. Note all photos are of other species, provided to illustrate the general characteristics of the genus.

Fontinalis antipyretica, showing aquatic habitat.
Photo by Bernd Haynold, Creative Commons license,
posted on Wikimedia Commons

The stiff leaves lack a midrib, and the cells are worm-like, but plump, and densely filled with chloroplasts. A few cells at the base of the leaf are larger, more squarish, and clear.

Spore capsules appear along the stem and are nestled within a cluster of specialized leaves, lacking an elongate stalk.
.
.

The flattened leafy shoots of Fontinalis sullivantii.  Photo by
Kurt Stuber, Creative Commons license, posted on Wikimedia
Commons.

The leaf tip of Fontinalis squamosa, showing the curvy,
worm-like cells filled with chloroplasts. 
Photo by Hermann Schachner, public domain, posted on
Wikimedia Commons

This species occurs throughout eastern U.S. and northern Europe.
It is found in northern Florida down to Hillsborough, Polk and Osceola Counties, though it has been sparsely collected.





Fontinalis may be confused with other aquatic mosses in Florida, but is distinguished from them by its lack of a midrib, the elongate, worm-like cells with thick walls, and the spore capsules that remain nestled within clusters of bract-like leaves.

Two other species have been collected sparsely in northern Florida: Fontinalis novae-anglae from the central Panhandle and possibly Orange County, and F. sphagnifolia, from central north Florida, with unconfirmed reports from Hillsborough and Polk Counties.  They differ in small, technical details.
.
.

The base of a leaf of Fontinalis antipyretica, showing larger,
clear, basal cells, captured nicely by Hermann Schachner, public
domain, posted on Wikimedia Commons.

Mosses of Central Florida 51. Leucodon julaceus

Leucodon julaceus (Hedwig) Sullivant (Leucodontaceae) forms colonies of erect leafy shoots arising from a branching stem system on tree trunks, logs, rock, and soil.

Photo by Scott Schuette, copyright MBG, posted on
Tropicos,  available under a Creative Commons License.

Leaves are short and scale-like, with inrolled edges, evenly distributed around the stem, and  lack midribs. Leaf cells are roundish-angular and largely smooth, but with some papillae on cells near the tip. When dry, the leaves press against the stem, resembling a tiny juniper twig.

Spore capsules are erect and egg-shaped, on short stalks arising from among specialized long, sword-shaped bracts, usually near the tips of the leafy shoots.

This species is found throughout the eastern U.S. and southern Ontario, as well as in Mexico and the West Indies.   It is found in northern Florida south to Hillsborough and Manatee counties.

It is somewhat similar to Schwetschkiopsis fabronia, but the latter is confined more to the bases of trees, and the leaves are "bumpy" throughout due to the translucent cell wall projections at the ends of cells.  Clasmatodon parvulus and Papillaria nigrescens are also similar but their leaves have distinct midribs.

Photo by Gerritt Davidse, copyright MBG, posted on
Tropicos,  available under a Creative Commons License.

Mosses of Central Florida 50. Cyrto-hypnum minutulum

Cyrto-hypnum minutulum (Hedwig) W. R. Buck & H. A. Crum (Thuidiaceae)
is a creeping, freely branching moss found on rotting logs, the bases of trees, and rocks.

A dried specimen identified as Cyrto-hypnum minutulum in
USF Herbarium (Griepenburg s.n., 4 Apr 1970, Highland
Hammock State Park)

The leaves are scale-like, with small roundish to squarish cells with multiple papillae on both sides.  As in other members of the Thuidiaceae, leaves on the main stem are larger than on the branches.  The midrib extends 2/3 to 3/4 of the leaf length. Spore capsules are asymmetrical and bent to the side.


The related genus Thuidium differs in that papillae are found only on the lower surface, and there is usually only one per cell.

This species is found throughout state but lacking in the southern Atlantic counties.  It is also found throughout eastern N. America, Europe, and south into South America. 

Also found in Florida, but very limited in distribution and distinguished on minor characteristics are:
C. involvens, southern Florida north to Volusia County, but with major gaps.
C. pygmaeum, 2 records: Jackson and Manatee counties
C. schistocalyx : Highlands, Miami-Dade counties

The species has previously been known as Hypnum minutulum or Thuidium minutulum.

Mosses of Central Florida 49. Stereophyllum radiculosum

The flat, dried leaves of  Stereophyllum radiculosum have a
conspicuously bulging midrib.
Photo by Juan David Parra, copyright MBG, posted on
Tropicos,  available under a Creative Commons License.

Stereophyllum radiculosum (Hooker) Mitten (Stereophyllaceae) forms thin, flat mats on the base of trees, exposed roots, stumps, logs, and limestone,  The distinctive flat leaves are attached uniformly around the stem (not flattened in a plane), elliptic-ovate in shape, and somewhat contorted when dry. The midrib is strong and markedly bulging, but does not reach the tip.  The leaf cells are small, roundish, and contains a single papillum (translucent bump). Spore capsules are erect to somewhat leaning,  asymmetrically egg-shaped, and arise from the bases of the leafy shoot on relatively short stalks (0.6-1.2 mm).

The leaning, egg-shaped spore capsule of Stereophyllum.
Photo by Juan David Parra, copyright MBG, posted on
Tropicos,  available under a Creative Commons License.

This species is widespread in the tropics; found in the U.S. only in Alabama, Texas, and Florida, where it occurs in the southern part of the state as far north as Citrus, Volusia, and Alachua counties.

Most other creeping species found on tree bases have somewhat flattened shoots, with leaves mainly on two sides of the stem, and capsules strongly bent to the side (Isopterygium or Haplocladium) or symmetrical and upright (Sematophyllum, Entodon).

Mosses of Central Florida 48. Schlotheimia rugifolia

The distinctive brownish shoots of  Schlotheimia rugifolia.
Photo by Juan David Parra, copyright MBG, posted on
Tropicos,  available under a Creative Commons License.

Schlotheimia rugifolia (Hooker) Schwagrichen (Orthotrichaceae) forms distinctive reddish-brown mats on logs, tree trunks, and branches.  The leafy shoots are more or less erect (extending away from attached base).  The flat leaves twist spirally around the stem when dry.

The narrow-ovoid capsules are usually erect, but bent in this
dry specimen. Photo by Juan David Parra copyright MBG,
posted on Tropicos,  available under a Creative Commons
License.

The leaves  of this species are elliptic, with short point at tip and have a slight rumpled (rugose) appearance. The midrib is strong, extending through to the short point.  Leaf cells are small, roundish, and smooth.  The spore capsules are erect and narrow-ovate in shape. They arise from the tips of the leafy shoots on elongate stalks,
.
Schlotheimia rugifolia  is found throughout the southeastern U.S. as far north as Virginia and Tennessee, and extensively in the New World tropics.  In Florida, it has been collected throughout the state but with gaps.  In particular, it has not been collected in any of the Atlantic coastal counties between Volusia and Miami-Dade.

This is one of the relatively few mosses found in Florida that occur relatively high on the trunks and branches of trees.  The dark, reddish brown coloring, the distinctive spiral twisting of the dried leaves and the longer capsule stalks will distinguish it from others, such as Sematophyllum,  Cryphaea, and Forsstroemia, in this habitat.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

Mosses of Central Florida 47. Aulocomnium palustre

Aulacomnium palustre (Hedwig) Schwagrichen (Aulacomniaceae) forms tufted colonies of upright leafy shoots on wet soil, marshes, swamps, and sometimes rocks.  It employs asexual propagation via small, modified, bullet-shaped or spear-point leaves clustered at the tips of elongate stems.

The upright shoots of Aulacomnium palustre form loose tufts
on wet soil. Photo by Robert A. Klips.

The leaves are elongate, with a prominent midrib, and gradually come to a point.  Leaf cells are small, roundish and papillose.  Spore capsules are bent to the side, and distinctly grooved, but are not commonly seen in our area, as asexual propagation is abundant.

A single tapered leaf of Aulacomnium palustre, showing
the prominent midrib and tiny, rounded cells.
Photo courtesy Western New Mexico Herbarium,
from the Gila Wilderness.

This species is found in every state and province of North America and throughout the northern hemisphere.  In Florida it has been collected in scattered counties in the northern half of the state, as far south as Polk County.

Mosses of Central florida 46. The genus Leskea

Leskea gracilescens growing on the side of a tree.  Photo by
Robert A Klips

Three of the four species of  Leskea found in North America have been reported from Florida: Leskea australis Sharp,  L. gracilescens Hedwig, and L. obscura Hedwig.   These are creeping, irregularly branched, mat-forming mosses, often with a reddish coloration, found on the bases or lower trunks of hardwoods and cypress trees, or on decaying logs. 

A leaf of Leskea gracilescens, showing the small roundish
cells with papillae (translucent light spots).  Photo by Kalman
Strauss, posted in the Consortium of North American
Bryophyte Herbaria database.

Leaves are ovate and pointed, with midribs that usually end before the tip.  Leaf cells are small, roundish, and papillose, and the spore capsules are erect and more-or-less symmetrical.

Only Leskea australis is known from central Florida.  It has been found throughout the state, but has not been reported from the extreme south, the western panhandle or the Atlantic coastal counties. Elsewhere, it is found throughout the southeastern U.S.

A leafy shoot of Leskea gracilescens with an inset of the
papillose leaf cells.  Photo by Robert A.Klips

The erect, symmetrical spore capsules of Leskea gracilescens.
Photo by Kalman Strauss.

L. gracilescens and L. obscura are both widely distributed in eastern North America, live in the same habitats as L. australis, and differ in minor ways. L. grascilescens has been reported from several counties in North Florida, elsewhere throughout eastern North America, and L. obscura only from Leon County.

Our Leskea species are similar to Haplocladium microphyllum, also in the Leskeaceae, but in Haplocladium the spore capsules are bent distincly to the side, and the leaf tips more drawn out into a narrow point.  H. microphyllum is also more likely found in soil, rocks or damp wood than on tree trunks. 

Pond edges, liverworts, and the earliest land plants

.The edge of a drying pond would hardly seem like a hospitable place for plant life.  During the wet season it is submerged, and during the dry season it can be completely dried out.  Yet for the few months when the shoreline is retreating, during winter and spring in Florida, the sandy soil between high and low water marks remains moist, and supports a variety of small, but quick-growing plants that complete their life cycles before being inundated again.

Some green plants can be seen along this drying pond edge, but
the most interesting residents can only be seen close-up.

I have previously reported on some ephemeral inhabitants of such pond edges, including the sundew, Drosera capillaris, and several mosses, including Rosulabryum capillare,  Physcomitrium collenchymum, and some species of Micromitrium.  I expect to report on several other mosses with similar habits in the coming months.
.
Recently, I was out walking in my neighborhood, and decided to revisit the same pond edge where I had collected Physcomitrium collenchymum in late March a couple of years ago. Now, in late January, I was quite surprised to find two completely different bryophytes,  Riccia cavernosa, and a species of Sphaerocarpos - both liverworts - but no sign of the Physcomitrium.  I shall have to pay attention over the next several months, to see if there is a succession of different species that might include later appearances of a  moss or two.

Riccia forms a flat, forking thallus, while Sphaerocarpos takes the form of a rounded mound of upright shoots.

.

Riccia cavernosa is a thallose liverwort, meaning its body is in the form of a flattened, forking ribbon. Riccia species tend to spread outward from a central point, forming a round disk, at least when young, One can see here that the thallus is heavily pitted, giving it a spongy texture.

Sphaerocarpos species form cushions of odd egg-shaped shoots.

.
.Finding these liverworts got me to thinking again about the very first land plants, which  most likely lived in a similar habitat, and were similar in growth forms to modern thallose liverwort like Riccia.  Phylogenetic studies indicate that the closest living relatives of land plants are in the green algal genus Coleochaete, which has a similar flat, disk-like growth form.  So it is easy to see an ancient transition from one to the other.

The green alga, Coleochaete, grows as
a flat sheet of tissue (A,B) that expands
outward from its center. The roundish
bodies are zygotes, that will eventually
undergo meiosis to produce zoospores (C).
from Essig 2015, after Haupt.

To be sure, some major adaptive innovations took place as green algae moved onto land, especially in their mode of reproduction. Coleochaete, like other green algae, produces zoospores through meiosis of the zygote, while all bryophytes have a diploid sporophyte generation that produces non-motile, dessication-resistant spores through meiosis of cells within a spore chamber or capsule.

[several interesting copyrighted photos of Coleochaete species can be seen at The Vine Tendril , and The Algal Web.

Also to be sure, Riccia is a modern liverwort genus, and I don't suggest that the first land plant were members of that genus that have been sitting around unchanged for 400 million years.  Most likely the ancestors of Riccia readapted to this habitat, as many others have over the ages, including the Sphaerocarpos, various mosses, and certainly the Drosera.

Part of that readaptation, is a shift back to the means of spore dispersal most likely practiced by the first land plants. In most bryophytes, the spore capsule is elevated by a stalk to facilitate wind-dispersal. The spore chamber of Riccia  is however very simple and lacks a stalk. It remains embedded within the thallus until the latter disintegrates, and the spores are then dispersed by water currents as the pond level rises, or on the feet of birds walking around in the mud.  Seeds of Drosera capillaris are probably dispersed in a similar way.

The sporophyte of Riccia is embedded within the gametophyte
thallus.  It consists of a thin wall (the capsule) containing a mass
of diploid tissue, which here has been transformed through
meiotic division into a number of hard-walled spores. From
Essig 2015, after Brown.

So the water-edge habitat has continued to host a dynamic community of ephemeral plants of similar growth form and reproductive habits since the very first colonization of the land.

Reference:
Essig, Frederick B., 2015. Plant Life - a Brief History. Oxford University Press.

.
.
.
.
.
.
.
.
.

Flowers, Compound Flowers, and Superflowers

In the simple inflorescences of Indigo
(Indigofera spp., Fabaceae) new flowers
are produced at the tip for an extended
period of time, opening first at base, and
lasting only for a day or two.  Many
inflorescences with more complex
branching patterns still open just a few
flowers at a time.

An inflorescence can be defined as an aggregation of flowers on a specialized shoot that lacks ordinary leaves.  The lack of full-sized, photosynthetic leaves is the key to defining an inflorescence, as opposed to a series of single flowers along a leafy branch.  Within an inflorescence, leaves may in fact be present, but they are either smaller than normal, specialized in shape, conspicuously colored, or all of the above.

One common type of inflorescence, is one in which flowers open up one or a few at a time, for an extended period.  Examples include lupines, snapdragons, gladioli, and foxgloves.  Such inflorescences are adapted to induce "repeat visitors" - insects, birds, or other animals that remember the location of the plants and drop by each day to collect nectar or pollen from freshly opened flowers. This is a behavior known as trap lining.


In such flowers, a common means of avoiding self-pollination, is for the stamens and pistil within each flower to mature on different days. For example, pistils may be active and receptive to pollen on the day the flower opens, with the anthers opening to shed pollen 24 hours later. The common Amaryllis follows this pattern.

A common means of avoiding self-fertilization within individual flowers is to have pollen shed on one day (left) and stigmas receptive on another day (right).  This effectively makes the flowers male on one day, and female on another day.  

Compound, or false flowers, such as those of the sunflower family or the spectacular poinsettias, are actually condensed inflorescences adapted to look like a single large flower to pollinators, but still opening their flowers a few at a time to attract trap-lining animals.

In the compound flower heads of the Sunflower
Family, such as this Ice Daisy, the tiny flowers
(visible as the yellow rod-like structures)
around the outside of the central disk open first,
to be followed by flowers progressively closer
to the center.

In other inflorescences, the flowers mature all at once to create a single, massive pollination event.  I like to call these "superflowers." Super flowers don't necessarily look like a single flower, but they behave like one.  In the most specialized of these kinds of inflorescences, flowers are unisexual, and the opening of the male and female flowers is offset, such that the entire inflorescence behaves as a single, short-lived flower. Examples of these are found most spectacularly in the Aroid and Palm Families.

The Titan Arum, Amorphophallus titanum,
blooming at the U.S. Botanical Garden in
Washington, D.C., posted on Wikipedia.

The Titan Arum, which makes the news whenever it blooms in a botanical garden, produces a gigantic inflorescence in which all the tiny flower buds are mature when the large bract, or spathe, opens to reveal them.  They follow a similar strategy as the amaryllis flowers above, with separate female and male phases.  In this case, all the female flowers are active first, followed by the male flowers in a day or two.  This avoids self-pollination, as insects arrive during the female phase, bearing pollen from another inflorescence.  They then leave during the male phase, freshly dusted with new pollen.

Unlike the simple inflorescences
mentioned at the beginning of this post,
palm inflorescences complete most of
their development within their large,
protective bracts.  No new branches or
flowers will form after the bracts open. In
many, such as this Rhopaloblaste, the
flowers will continue to expand for a
while, and may open over a prolonged
period of time in order to attract trap-
lining insects. 

The inflorescences of palms, despite being rather large, appear to be fairly simple assemblages of small flowers, yet some of them behave in much the same way as the Titan Arum.  I made my discovery of these palm "superflowers" as a graduate student, first in Costa Rica, and later in New Guinea.

After waiting for a few hours at the edge of a swamp in Costa Rica, I found that inflorescences of a species of Bactris opened abruptly at dusk, displaying unopened male flowers and active female flowers nestled within them. The inflorescence emitted a musky odor, which attracted a variety of small flies, bees, and beetles. The male flowers opened to release their pollen 24 hours later. I observed the same thing in another species of Bactris later.

A year later, I was in Papua New Guinea and observed a nearly identical process in a species of Hydriastele.  I was able to get more detailed pictures of male and female flowers, along with their insect visitors, which I share below.

Though it consists of a number of branches, the inflorescences
of Bactris guineensis behave the same as that of the Titan Arum. Female
flowers, hidden among the larger male flowers, are
receptive to pollen soon after the large, fibrous bract opens.

When the bract of a Hydriastele microspadix inflorescence splits
open, the flowers are all mature, and arranged in triads of
two large male flowers with a tiny female flower between them.  

   

When the flowers are first exposed, the stigmas of the tiny female flowers, seen at the left between pairs of male flowers, are exposed, sticky, and receptive to pollen..  The male flowers (right) only open 24 hours later to release their pollen.  During the female phase, tiny flies and weevils are already present, attracted by the scent of the unopened male flowers.

One further example from the palm family comes from the mangrove palm, Nypa fruiticans, which lives in brackish water around river deltas and estuaries throughout the old world tropics. I spent a day observing them.  Here the female flowers, which are rather bare and uninviting, are borne in a tight globose head at the top of the inflorescence. Male flowers are borne on dense orange-colored spikes below the female flowers.  Brief observations suggested that pollination is accomplished by small flies that land first on the female flower heads, and then crawl down to the spikes of unopened male flowers where they lay their eggs.  Larvae develop within the spikes, feeding on the unopened male flowers, and mature in a few days.  When the new adult flies emerge, the male flowers have opened and are shedding pollen.  The flies are then covered with the sticky pollen and fly off to start a new cycle on another Nypa inflorescence.

Female flowers of Nypa form a dense globose head (left), and appear to provide no nutrition for insects.  The dense male spikes (right), however, provide a place for fly larvae to develop as they feed on the tissues of the unopened male flowers.