Mosses of Central Florida 22. Leptodictyum riparium

Leptodictyum riparium (Hedw.) Warnst. (Amblystegiaceae) is the most common aquatic moss in central Florida.  It is commonly found growing submerged on rocks along rivers, typically in thick mats, or exposed on rocks or tree bases close to water.  Other aquatic mosses include some forms of Hygroamblystegium varium, and two species of Fissidens, which are much less common.  Aquatic species of Sphagnum are confined to north Florida.  Amblystegium serpens may be in our area, but voucher specimens need to be verified.

Leptodictyum occurs in thick underwater mats. The stems are long, straight and sparsely branched.  The leaves are spread stiffly in one plane, even when dry.  From Franck 3314 (USF).

Leptodictyum is distinguished from other aquatic mosses by its long, straight, sparsely branched stems, with leaves extending stiffly from the sides of the stem (in more or less one plane). The leaves remain more or less stiff when dry, by may be somewhat rumpled.  It rarely produces sporophytes in our area.   Hygroamblystegium has shorter, more branched stems that tend to be more curved when dry, and shorter, more triangular leaves distributed uniformly around the stem.  It is also more often found with sporophytes.   Species of Fissidens are distinguished by their doubled leaves.

The leaves of Leptodictyum have a distinct midrib, but which
does not quite reach the tip. From Franck 3314 (USF)

The leaves have a distinct midrib, and the leaf cells are generally elongate with thin, inconspicuous walls. Sporangia, when present, are somewhat curved and asymmetric, but none have been found among the specimens at USF.

The leaf cells are somewhat elongate and tapered at the ends,
but do not stand out sharply under the microscope. From
Lassiter et al 559 (USF)

Mosses of Central Florida 21. Introduction to the genus Fissidens and F. pallidinervis

An image of Fissidens taxifolius illustrating the basic flattened shoot structure
 of the genus, with doubled leaf.  Image by Ralf Wagner.

According to Flora North America, twenty species of the genus Fissidens  occur in Florida.  By definition all members of this genus have a peculiar flattened leafy shoot with double leaves.  At each node or insertion point along the stem there is a full-sized leaf and a smaller leaf. The leaves are attached sideways to the stem, creating a flat, frond- or feather-like shoot. The leaves do, however, tend to curl when dry. At the bottom of this post is my effort at a simplified, short-cut key to the species.

As in most large moss genera, the individual species are distinguished by technical characters, mostly microscopic features of the foliage, and can only be definitively identified by experts.  However, there are some shortcuts that can help narrow down the choices in the relatively small number of species found in Florida (there are 450 species world-wide, 37 in North America), including the habit and length of the capsule stalks.

A dried specimen of Fissidens pallidinervis in the
herbarium at USF.  Note the relatively short stalks
of the capsules (sporangia), and the twisted  dry
leaves.

Fissidens pallidinervis Mitten is relatively common in Florida, from the panhandle to the keys.  It is documented elsewhere only in Louisiana, but might be expected to show up in southern Georgia, Alabama, and Mississippi.  It typically occurs in moist areas at the bases of trees or on decaying logs, but found in our area also on shell mounds.

 From the other 19 species it is distinguished by a combination of  details, mostly of the leaves.  The cells of the leaf, on either side of the prominent midrib, are tiny, roundish and papillate, which means they bear small, translucent bumps on their surface.  This species has 2 or more such bumps, while some species have only one, and others have none (are completely smooth).

A single shoot from the specimen above.  Note that three
sporangia are rising from a single point on the stem.

The leaf cells of  Fissidens are small and roundish.  In this view,
the papillae can be detected as tiny bright spots of lighter
green.

The shoots are tiny, about _mm long, and each may produce 2 or more sporangia near the shoot tip.  The stalks of the sporangia are relatively short, compared to other members of the genus, about 2 mm long. Only F. leptophyllus is in the same range, but in that species there is a clear limbidium ( a row of scleried-like cells) on the lower margin of the leaf, the leaf cells bear only 1 papilla, or are only rounded.  While others are variable, with stalks sometimes in this range, they are usually longer.
















Provisional short-cut key to the species of Fissidens in Florida.

1_Aquatic
2_Sporangium stalks .5-.6mm………….F. fontanus
2_Sporangium stalks .7-1.5 mm……….F. hallianus
1_Terrestrial
3_Sporangium stalks less than 6 mm long
        4_Occurring on bark, tree bases, not soil-specific
                   5_Sporangium stalks to 2 mm long
                         6_Leaf cells rounded or with one papilla; bases of trees, cypress swamps
                                                       …………………..............……..F. leptophyllus 
                         6_Leaf cells with several small papillae; bases of trees.....…F. pallidinervis 
                   5_Sporangium stalks 3.5-5 mm long
                        7_Leaf cells smooth to 1 papillate
                              8_Leaves up to 6 pairs, markedly toothed, cells 1-papillate; on bark,                                                                     logs, cypress trees…....F. serratus 
                           8_Leaves up to 25 pairs, not toothed, cells smooth to rounded; Bark, tree                                                             bases, rotten wood .................….F. santa-clarenis 
                              8_Leaves up to 10 pairs, minutely toothed, leaf cells smooth to bulging;                                                               bases of trees, soil............................…....... F. amoenus 
                      7_Leaf cells with several small papillae; leaf apex ending in a sharp, clear                                                               cell, on soil, banks, roadsides,  uprooted trees …….F. elegans
           4_Occurring on calcareous soil or rocks
                    9_ Found throughout Florida 
                            10_ occurring primarily at bases of trees; leaves in a pinnate pattern, up to 28 pairs
                                    ..........................................F. subbasilaris              
                            10_ on moist limestone in ravines; Leaves in a palmate pattern, up to 12 pairs .
                                    ......................................................F. zollingeri 
                    9_Found in north Florida only; on wet soil or rocks along streams, 
                            11_ Found in Florida panhandle *;soil, rocks and stones in shaded  ravines                                                               ………......................................................……..………..F. pellucidus 
                           11_Found in northern peninsular Florida, stems branched,........F. obtusifolius
                          11_Found in northern peninsular Florida*, stems unbranched …F. minutulus 
3_Sporangium stalks to 10-11 mm long
                            12_Capsules somewhat asymmetric, 1 mm long; occurring on bare, often clay, soil 
                                  .......................................................................................….. F. bushii 
                            12_Capsule 1.8 mm long; occurring at bases of trees, roots…......... F. dubius 
                            12_ Capsules symmetric; bases of trees, soil, logs .................…….F. bryoides 
3_Sporangium stalks to 15 mm long
                          13_ Capsules to 2.5 mm long, symmetric; on calcareous soil, north Florida only
                                                  ................................………………..F. polypodioides 
                          13_ Capsules to 1.5 mm long, asymmetric; seeps, moist rocks, soil;  central Florida
       3_Sporangium stalks to 25 mm; on seeps and rocks with dripping water……….. F. adianthoides 
       3_Sporangium stalks to 17 mm; on moist soil .humus, .rocks ................................…….. F. taxifolius 

*Questionable records of F. pellucidus and F. minutulus in Collier County, far removed from their north Florida ranges

Mosses of Central Florida 20. Polytrichum commune

[For other mosses in this series, see the Table of Contents]

Polytrichum commune Hedwig (Polytrichaceae) is the giant among Florida mosses, with stems up to 10 cm. long. The stems are upright, with numerous stiff, narrow leaves.  It is not common in Central Florida, but when  found,  it is usually in extensive, dense colonies.

A colony of Polytrichum commune growing near Ft. Lonesome, Florida, in
Hillsborough County.  Many of the shoots in this picture bear clusters of sperm-producing antheridia at their tips. Photo by Steve Dickman.

There are seven species of Polytrichum found in North America, out of 70 worldwide.  P. commune is found throughout North America, Eurasia, New Zealand and Australia.  I recently found it in Taiwan.

The single specimen with sporangia at USF
shows the impressive dimensions of
Polytrichum commune. It comes from
northern Florida.

The leaves in the Polytrichaceae are unusual in consisting mostly of a massive midrib covered with vertical sheets of tissue arising from the upper surface.  This makes viewing the leaf anatomy more difficult, but the edges of the leaf in Polytrichum commune are lined with distinctive sharp teeth.  The bases of the leaves flare out into a broad sheath, and only here can one see the shapes of the cells.

 The upper surface of a Polytrichum leaf is covered with vertical,
blade-like sheets of tissue, here seen in cross section.
Photograph by Kristian Peters; Creative Commons
Attribution-Share Alike 3.0"

The midrib of Polytrichum commune is thick and fills the entire
blade.  Distinctive hard teeth line the edges.

The base of the leaf flares out into a thin
sheath, where the cells can be seen to be quite
narrow and elongate.

Evidently, Polytrichum commune rarely produces sporangia in Central Florida, as we have   We have o such specimens in the USF herbarium.  I can only speculate on the reasons for this.  In order for sporangia to form, sperm cells must swim from the tip of a stem where they are produced to the tip of a stem where eggs are being produced.  The large size of Polytrichum commune would make the conditions where this process could take place rather rare, particularly in the relatively hot and dry climate of Central Florida.  The species is at it's southern limit here. The one specimen with sporangia I've seen was collected in northern Florida.

 In addition, the colonies of Polytrichum are unisexual - they produce either sperm or egg, so two colonies of different sex must intermingle for sexual reproduction and spore-formation to take place.  The size of the plants would add to the difficulty of such mingling, and it is possible that some colonies in our area were established by a single spore, and hence unisexual.  In the photo  of living plants in Hillsborough County above, the colony appears to be all male.

Incidentally, in the process of researching this species, I found another excellent blog site for mosses from the University of British Columbia. Check it out for general information about the different groups of mosses.

The nearly forgotten art of comparative plant anatomy 1. Wood

The study of human anatomy is an obviously essential subject for all medical students, and most of us  have a basic idea of where our liver, lungs, large intestine, etc. are located - important information for identifying and fixing health problems.

Comparative anatomy, on the other hand, is about the differences and relationships among different organisms.  Most of what we know about dinosaurs comes from their bones.  Dinosaurs with bones of similar shape and arrangement are considered closely related to one another, and so we can reconstruct not only individual dinosaurs from their bones, but also phylogenies (family trees) and classifications.  Bones, by certain structural characteristics can also give us clues about what dinosaurs ate and whether they were warm-blooded or cold-blooded.

The comparative anatomy of plants has been put to similar use, helping us to interpret fossils, as well as to help identify and classify living plants.  Wood anatomists can identify the genus, and often the species, of a particular wood sample, and this is put to use by archaeologists and forensic scientists as well. How often has Abby from the hit TV show NCIS led her colleagues to a crime scene in the woods, by identifying wood, leaf, or pollen fragments from a murder victim's body or clothing?  Wood scientists, like those at the USDA's Center for Wood Anatomy Research, maintain a library of wood samples, including thin sections for microscopic examination.

The academic study of plant anatomy has, however, been in decline for decades (see Dengler 2002) .  This is partly because of the decline in botany courses in general, but also because of the need to make room in biology curricula for the massive amount of new material in ecology, cell biology, genetics, DNA sequencing, genomics, etc.  Plant anatomy in particular is considered by many to be old-fashioned, having largely been "done" in the 19th and early 20th centuries.  The study of plant anatomy, traditionally taught by looking through microscopes at prepared slides and making drawings, is admittedly tedious, requiring an interest and passion that is rare among most modern students.  To be of any use, one must acquire a broad knowledge of the different cell types and tissues of plants, and to recognize them as they vary in form from one species to another.  So it is a type of training, involving "memorization" of terminology and structural detail that is incompatible with the emphases on principles, theory, and experimental methodology in modern biology courses.  Moreover, the specialized equipment and skilled personnel for making thin microscope slides is rarely found anymore in biology departments.

Yet there is so much that we can learn still by comparing the anatomical structure of different kinds of plants.   Aside from practical applications in forestry, forensics and archaeology, it is still important in understanding the relationships and adaptations of plants in general.  The next few postings will illustrate some of this.

Let's start with the obvious: the structure of wood.  We will note first that plants are simpler that animals, and in plant anatomy, we're talking more about tissues than organs, actually more about what they would call histology in animal studies.  A tree trunk, composed of wood and other tissues is a single organ, so it is the different layers of tissues and cell types that determine how it functions, how it is adapted for its environment, and how one species differs from another.

True wood, as found in gymnosperm and dicotyledonous trees, consists of layers of xylem tissue added each year that increase the thickness of the trunk, branches, and roots.  Each layer is made up of water-conducting cells, and various sorts of supportive cells ranging from thin-walled parenchyma to thick-walled fibers.

The wood of  balsa (Ochroma pyramidale) is soft and light because the supportive

tissue around the large vessels is thin-walled.

 Photo from  Curtis, Lersten, and Nowak, University of Wisconsin, Stevens Point.

As an extreme example, the wood of the balsa tree is radically different from from that of a
teak tree, allowing us to not only recognize each under the microscope, but also telling us something about their different life styles.  Balsa is a fast-growing, short-lived tree that colonizes disturbed areas in forests and then gives way to longer-lived trees with more durable wood like mahogany or teak.  The density of wood is determined primarily by the abundance and distribution of fibers around the thinner-walled water conducting cell.  Durability is also enhanced by preservative chemicals secreted into the wood.
Among the great variety of dicotyledonous trees, there is great variety in the shape, arrangement, and abundance of these different kinds of cells. This variation has resulted in some woods being superior for fine cabinetry, others for resilient baseball bats (ash), durable bowling pins (hard rock maple), or strong shovel handles (hickory or ash). The wood database contains extensive images and data concerning the characteristics and uses of different kinds of wood.

In the tropical hardwood teak (Tectona grandis), the supporting tissue around the large vessels is made up mostly 
of narrow, thick-walled fibers.

In temperate trees, such as this red maple (Acer rubrum) the annual growth rings of wood
(horizontal bands) are marked by distinct boundary layer of smaller, thick-walled cells laid down in the Fall,
something you don't see in tropical woods.  The vertical bands of cells are made up of living
parenchyma cells, often with thick secondary walls.

The flexible woody stems of grape vine (Vitis spp.) contain exceptionally wide
rays of parenchyma tissue that separate the narrow wedges of vessels and other
supportive tissues.

The simpler, softer wood of conifers, like pine or fir, consists mostlyof a single type of relatively narrow cell, the tracheid, which combines strength with water-conducting ability.  As angiosperm trees diversified, ancient tracheids diverged evolutionarily into two kinds of cells. Strength and density functions were taken on as some cells developed thicker walls, becoming fibers.  Water-conduction was focused in other cells that remained thin-walled, but became wider, and shorter: the  stacked cylindrical cells of the vessels.  There is also a network of parenchyma cells that permeate the wood as flat rays or vertical strands

The wood of conifers in general is very simple, consisting mostly of  tracheids, which conduct water as well as provide
structural support for the tree.  The band of narrower, thick-walled cells running across the bottom of the image is a
boundary layer, marking the slowing and  cessation of growth in the Fall.  Above it, the uniform mass of cells produced
in the Spring and Summer.  

In pines and some other conifers, the uniform growth of
cells is interrupted by large resin canals.

In monocots, the ability to produce layers of wood was abandoned as their ancestors evolved underground stems and clonal lifestyles.  When some of them became trees again, as in palms or bamboos, their water-conducting cells remained in separate vascular bundles scattered throughout the interior of the stem and surrounded by dense sheaths of fibers.

The dense, wood-like property of a palm trunk is provided by dense masses
of fibers, not layers of xylem laid down in rings. In this preparation, the walls
of the fibers are stained green rather than the usual red.

The rigid wall of a hollow bamboo stem is filled with  fibrous bundles.

On-line wood anatomy resources:

Curtis, Lersten, and Nowak, Photographic Atlas of Plant Anatomy, revised 2015

Schoch,W., Heller,I., Schweingruber,F.H., Kienast,F., 2004:  Wood anatomy of central European Species.

Meier, Eric,  The Wood Database.

Inside Wood, North Carolina State University, developed by Elizabeth Wheeler and others.  This site is more technical, and contains the most extensive library of wood anatomy images.

Mosses of Central Florida 19. Rosulabryum capillare and R. pseudocapillare

Although the leafy shoots are buried within a
different moss, the presence of this Rosulabryum
is evident by the abundant and very large sporangia,
or capsules.

[For other mosses in this series, see the Table of Contents]

Like many other groups of mosses, those in the family Bryaceae are difficult to identify.  The available keys are highly technical and difficult to for non-specialists to follow.  The two species of Rosulabryum are presented together here because of those difficulties. R. capillare (Hedwig) J. R. Spence (Bryaceae) is common throughout North America, including Canada and our western states.  According to Flora North America, it is not found in Florida, but the related species R. pseudocapillare (Besch.) Ochyra takes its place.  However, numerous collections made in Florida have been identified by experts as R. capillare, along with many labeled as R. pseudocapillare.

Here, some leafy shoots have been isolated.
They are upright and radially symmetrical,
and the stalks of the sporangia arise from
the bases of the shoots. Sporangia are
symmetrical, cylindric and nodding.

Rosulabryum pseudocapillare typically has numerous thread-like reproductive structures (filiform gemmae) in the upper leaf axils, at least when not producing sporangia, while those of R. capillare do not.

Rosulabryum is common in Central Florida  and produces abundant nodding, nearly cylindrical sporangia (capsules) in the springtime.  It can be found most readily in wet soil at the margins of receding ponds, forming mounds of upright stems. It might almost be considered weedy, as it also pops up readily in pots containing wet, organic soil, and in wet soil along sidewalks.

Rosulabryum was formerly included in the genus Bryum, having been segregated out on details that are hard to follow in the formal keys.  In Florida, only Bryum argenteum remains in the original genus, which incidentally gives its name to the entire clade of non-vascular plants we call Bryophytes.  B. argenteum differs from Rosulabryum in its more compact growth form, its grayish coloration, and leaves that press flat against the stem when dry.  The dried leaves of Rosulabryum twist around the stem.

Nearly ripe sporangia of Rosulabryum
are cylindrical and bright green.

The leaves of Rosulabryum become twisted when dry.

The other segregate from Bryum found in Florida is Gemmabryum, but it apparently forms sporangia less often than Rosulabryum, relying more on asexual bulbils and gemmae for reproduction.  When sporangia do appear, they are more egg- or pear-shaped.  The leaves in Gemmabryum are also said to be pressed flat against the stem when dry, as in Bryum argenteum, rather than being twisted like in Rosulabryum.

Leaves of Rosulabryum, like other members
of the famly, have a strong midrib (or costa).

Leaves in the Bryaceae have a strong midrib, and the leaf cells are mostly large, more-or-less rectangular or rhomboidal in shape, thin-walled, and with numerous distinct chloroplasts.  This, along with the upright, mound-forming stems and the nodding sporangia, makes members of the family fairly easy to recognize, though the genera and species are more difficult to distinguish.

Leaf cells of Rosulabryum, are thin-willed,
revealing many distinct chloroplasts within.

The leafy shoots of Rosulabryum peudocapillare produce
many thread like reproductive structures called filiform gemmae,
while those of R. capillare rarely do.

Teaching biodiversity

The biodiversity crisis is one of the most pressing issues facing mankind.  But what is biodiversity? How does it come about?  Why are there so many kinds of organisms?  And what are the consequences of lost biodiversity?  Sounds like something we could devote a whole course to in our undergraduate curriculum.  But when we do, it's usually only to a handful of upper level ecology students.  What about our general student population, all those pre-meds we educate, or the public at large?  We would like our general citizenship to be well-informed on the dimensions and importance of biodiversity, but at the very least, our own biology majors should be not only informed, but also advocates.  How are we all doing on that?

The one, and sometimes only, opportunity we have to reach all of our undergraduates is in freshman biology.  A second opportunity comes if we can get students to take an organismal course like introductory botany, invertebrate zoology, mycology, or entomology.  The following suggestions apply equally well to all such courses, but I will focus on Freshman Biology, which in my university is taught in two separate one-semester courses: Cellular Processes and Biodiversity.  We use one of the standard large college texts for introductory biology.

The second course, Biodiversity, sounds good, but it has an ambitious amount of material to cover.  It is divided roughly into three sections:  Evolution, Diversity, and Ecology.  The section on Diversity, which might get five weeks out of the semester, is usually a rapid march through the kingdoms and phyla.  Plants usually only get 1-2 weeks of discussion in lecture,  fungi even less, more often than not by an instructor who has not had much training in botany or mycology.  The sections on animal and plant biology in the textbook are largely ignored in our courses, but they contain much additional information that can and should be brought in to help elucidate biodiversity issues.

The net result is that after this course in Biodiversity students have only a superficial introduction to diverse kinds of life, and have no real understanding of why there are so many different kinds of organisms.     The coherent biodiversity message will only  emerge if an experienced and skillful instructor is motivated to do so, and willing to synthesize material from different parts of the book.  The more we can integrate our discussions of evolutionary and ecological principles with questions about biodiversity, rather than treating them as three separate topics, the better. This is a tall order for the limited time available in a freshman course, so you have to choose your battles carefully.


Here are some ideas, with links to my blog essays that discuss these topics in detail:

1.  Think in terms of  adaptation.  This is the key concept  that links evolution, ecology, and biodiversity together. Adaptation is what results from the evolutionary process, it defines how organisms interact with their environment, and it is what differentiates the distinctive lifestyle of one organism from another.  Organism A is different from organism B because, since their common ancestry, they have had different adaptive histories, and have diverged into different lifestyles.  They have come to live in different environments  or to survive in the same environment in different ways.

The flowering structures of skunk cabbage are
adapted for getting a jump on the spring
flowering season by generating sufficient heat
to melt their way through the remaining snow.
Photo by Sakaori, from Wikimedia Commons

2.  Every feature has a function.  Everything we see in an organism, from the shape of a leaf to the color of tail feathers in birds, has some adaptive function, or did sometime during the adaptive history of the organism.  There is a story for every characteristic feature of a species.  Pollination biology presents many attention-grabbing examples.

3. "What good is half an eye?" Make use of some of the great questions posed by anti-evolutionists (though answered wrongly by them).  By focusing  on a single topic, like vision, one can trace the origins of light detection in bacteria, through the simple eyespots of protists, the simple eyes of flatworms, and then the diverse kinds of eyes found in cephalopods (squids, octopi), insects, and vertebrates.  There never was half an eye, always light detecting systems that became more complex and varied over time.  We can even bring in plants with their light-detecting systems  involved in phototropism (bending toward light), and photoperiodism (determining when plants bloom.)

 4. "If humans evolved from apes, why are there still apes?"  Another great question that illustrates the diversifying nature of evolution.  From the common ancestor of chimpanzees and humans, the chimp lineage continued to hone their adaptations for life in the forest, while human ancestors adapted for life in the open savanna, with skeletal changes that allowed them to stand upright and walk comfortably on two legs.  Students really take interest in human evolution, and so it is worthwhile to spend time on it.

Animals are eating machines, with mouth and eyes at the
front end, and locomotory organs along the side. Their
food resources are in compact packages - other organisms -
that they digest internally.

2. "What is the difference between plants and animals?"  I always began my botany classes with this question.  Almost everyone starts with the observation that plants are photosynthetic and animals eat stuff. True as it is, it is only  the beginning. The next thing that might come up is that animals move and plants don't.  Then as an instructor, you ask "why?"   Asking why is not to be anthropomorphic.  It's really asking "what are the adaptive advantages for plants not to move?"

Plants are anchored in one spot and create
branched systems above and below ground for
gathering diffuse resources. Meristems continually
create new roots, twigs and leaves.  And materials
are transported internally by manipulating water
pressure.

One can then direct the discussion to the stationary, indefinitely branching body of a plant, adapted to create  an expansive antenna system for gathering light and other diffuse resources, in contrast to the discrete bodies of animals with mouth and sensory organs at the front end, and locomotory organs along the sides.  One must then discuss the system of meristems that enable varied plant architectures built of repetitive leaf-bearing units, and the hydrostatic nature of plant cells and plant processes that substitute for muscular activity in animals.



3. Why are there no moss trees? Everyone knows, at least by the time they get to college, how animals make babies.  The varied equipment and various strategies for getting sperm and egg together are a wonderful theme for exploring animal diversity, but how do plants do it? Plant (or fungus) reproduction, however, is always a challenge.    If you're stuck in one spot, and a potential mate is 50 meters away, how do get your sperm to her?  The astute student will immediately shout "pollen grains."  But how many know that there are actually sperm cells produced within pollen grains, and that pollen grains, and the structures that house the eggs are actually tiny. haploid individuals?

Eggs and sperm cells in ferns are produced on a
separate, independent, short-lived plant
(gametophyte)  that develops from a spore
released from the main plant (sporophyte).
Meiosis occurs during the production of the
spores, rather than in the production of gametes.
From Haupt, A. W. 1953. Plant Morphology.

That brings up the dreaded life cycle.  Students hate them and instructors who don't know their significance tend to skip over them. Memorizing a life cycle does not explain why life cycles exist.  What is the adaptive value of having separate tiny haploid plants to bear the sperm and eggs?  The place to begin is with ferns.  There is adaptive value in breeding with distant, genetically different individuals.  That would be impossible if the fern plant produced sperm and egg cells directly, as sperm cells on dry land can't get very far.  So instead, it produces spores, which can easily disperse over great distances.  Often enough, spores from genetically different ferns land together, These spores then germinate to produce tiny gamete-producing plants that can breed with one another.  The fertilized eggs then develop into a new fern plant.

The leafy, long-lived phase of a
moss life cycle is the egg and
sperm producing gametophyte.
The simple sporophytes consist
only of a single sporangium and
its stalk, which develops from
the fertilized egg, and which
remains attached to the gametophyte
plant for its short existance.

The answer to the question about moss trees is that a moss is actually a gamete-producing plant, and must remain small so it can mingle with genetically different plants for successful reproduction.  Mosses lack independent spore-producing plants, having instead small diploid spore-producing structures that emerge directly from the fertilized eggs, but remain attached to the parent plant. Thus there is no part of the moss life cycle that can get really large.

 Pursuing these sorts of discussions is of more value than memorizing the characteristics of all the phyla of invertebrates, or the differences between club mosses and horsetails.  Horsetails can be brought in, however, as an early example of the kind of multiple elongating (intercalary) meristems used by bamboos for their rapid growth in height.(convergent evolution).  Ultimately, we can try to understand why there are so many kinds of plants, and how to avoid the extinction of all those species.

Endangered plants, the population bomb, and politicians running for office

Cycads are gymnosperms that have survived from the dinosaur
era.  They are increasingly at risk of extinction from poaching
and habitat loss.  These Encephalartos brevifoliolata
from South Africa are already extinct in the wild.
Photo by Piet Vorster, CC BY-NC-SA 4.0 .

The cycad Encephalartos brevifoliolata is an endangered species.  In fact, it is already extinct in the wild, surviving only as a few specimens that had to be moved to a secure location.  Habitat loss and poaching for the horticultural trade threaten many species of cycads. These ancient gymnosperms are among the relatively few plants that exhibit complete gender distinction. Individual plants are either male or female, and as it happens, the surviving members of this species are all male.  Even if there were a female among them, their gene pool would be severely restricted, and future generations would be highly inbred.  With such limited genetic variation, recessive genetic defects are more likely to be expressed, and the entire population much more likely to succumb to disease or environmental change. Cacti, orchids, and carnivorous plants are other groups particularly threatened by poachers, but many other kinds of plants are equally threatened by loss of natural habitats to logging, agriculture, and "suburbanization."

More familiar examples of this are of animals.  Aside from the loss in numbers and other threats, concerns about the rare Florida panther center around its minimal genetic variation.  Animals in more vulnerable 

regions, such as rhinos, tigers, snow leopards, great apes, etc. face more immediate threats, but those that survive, possibly only in zoos, will face the genetic inbreeding problem as well.   Under such genetic constriction, the future of these species is in question.

Carnivorous plants around the world, like this Sarracenia rubra
in north Florida, face habitat loss and poaching by hobbyists.  

According to the Center for Biodiversity, a significant percentages of the Earth’s plants and animals are at risk of extinction, including 50% of the species of primates.  An estimated 1000 species of plants and animals that have already gone extinct under the watch of humanity (see The Extinction Crisis).  Again, animals are more familiar – dodos, passenger pigeons, etc., but 123 plant species have also been officially documented as going extinct in historical times (according to Wikipedia).  This is not to mention also the fact that possibly only half, or less, of the species of living organisms on this planet have even yet been documented by scientists, so we don’t know many that have already gone extinct or are endangered.  The present threat to the existence of organisms of all types, called the biodiversity crisis, could become a mass extinction greater than that in which the dinosaurs became extinct.

What are the consequences to the global ecosystem of plant and animal extinction?

This Dendrobium bracteosum was salvaged from the

branches of a tree felled for the timber industry in

Papua New Guinea in 1971.  Though widely cultivated,
what the fate of this species in the wild is today,
I do not know.

The loss of any species disrupts and simplifies the complex interactive web of life around us.  A plant that goes extinct may be the host or food source for dozens of other organisms, and likewise, an animal may be an important link in the food chain or a predator that keeps other animal populations in check.  Destabilized ecosystems are less productive, subject to wild fluctuations, suffer soil erosion, and become overrun by tough, weedy species of no use to anyone.  Loss of forests and the poisoning of photosynthetic algae in the oceans diminishes the replenishment of oxygen in our atmosphere, and the removal of carbon dioxide.

Why is all of this happening?  We might point to lack of regulation and enforcement, poor land management and forestry practices, human greed, and maybe the erroneous belief by many collectors that they are helping "save" rare species by growing them in their backyards.  But clearly, the growing human population, with its expanding demand for farmland, wood, clean water, and other natural resources, is directly related to the loss of natural habitats required by the others species we share this planet with. 

In 1968, Paul Ehrlich and his wife Anne (uncredited at the time) published The Population Bomb (Sierra Club/Ballantine Books, ISBN 1-56849-587-0), which warned of the dire consequences of uncontrolled and excessive growth of the human population.  They predicted widespread famine and other disasters as early as the 1970’s. The predictions were based on sound biological principles.  Every species tends to increase in numbers, because individuals have the potential to produce many more offspring than needed for their replacement.  In a balanced ecosystem, populations of each species are kept in check by limited food supply and other essential resources (e.g. nesting sites for some animals), by disease or predators, or by fouling their own environment.

The organ pipe cactus, Stenocereus thurberi,
is one of hundreds of cactus species that are 
endangered.
Photo by Lars Hammar CC-BY-NC-SA 2.0

Though the Ehrlichs’ may have been off on the timing of some of their predictions (see their recent review article), the need to bring the human population under control is accepted by scientists and humanitarian groups in general.  The principle objections come from people who fear draconian governmental population control measures, or the impacts of no-growth economics.  Again, I can’t do a full review here, but a simple google search for either “overpopulation” or “underpopulation” will bring up a number of links (yes there are people who believe that we, at least in some countries, are underpopulated). 

Before I move on though, I must remark that while the world may not appear to be overpopulated from the biased perspective of affluent America, the 760 million people in the world who are currently undernourished, the 8,350,000 people who die of starvation each year (Worldometers), or the perennially impoverished people of Haiti who just suffered devastating losses from Hurricane Matthew, might beg to disagree.

Expanding numbers of desperately poor people encroach upon national parks and other wildlife preserves to find a means of livelihood, further endangering species and disrupting natural ecosystems.  Maybe we can support more people on this planet, but only by converting ever more wild land to food production.  As we attempt frantically to increase our food supply, we cut down more forests, irrigate deserts, and even encroach upon estuaries and marine habitats.  We also use more fertilizer, pesticides, hormones and antibiotics, keep animals in small cages, and continue to genetically modify our food crops. Watch the streaming statistics on Worldometers for a few minutes, you can also see that almost 4 million hectares of forest have been lost this year, and over 5 million hectares of soil have eroded away, along with other disturbing numbers that continue to increase.  And despite all the technological advances, if population continues to grow, we'll back to where we started, but with even more starving people, less wild land, and fewer species of plants and animals.

Adding the problems of air and water pollution, nuclear leaks, toxic waste dumps, and climate change, we are not only fouling our own nest, but that of wild plants and animals as well.  Coral reefs are dying around the world, as well as forests in the Appalachians and California mountains, honeybees are being poisoned, and tree frogs are dying from the effects of pollution.  As sea levels rise and ice caps disappear, not only are polar bears threatened, but also coastal estuarine communities (breeding grounds for many commercial fisheries), sea grass beds and lowland swamps, not to mention the billions of people who live in coastal cities. 

The human suffering is  front and center in our collective humanitarian consciousness, and protecting rare species may seem to be a luxury for the benefit of the affluent, but they are actually both manifestations of the same central problem.  Bringing population growth under control will benefit both people and biodiversity, and the sooner we do it, the better.

Instead of just trying to keep up with increasing population size with ever more technological fixes, maybe we should be asking how many people can the Earth sustain while providing a just and equitable distribution of resources to all of our inhabitants, and while maintaining a viable, biologically diverse ecosystem with which to sustain ourselves.  I would think that, just maybe, we already have enough people on this planet, maybe more than it can sustain cleanly with renewable resources over the long run. Perhaps even a small decline would be helpful in taking care of everyone already here and getting back into balance with our natural ecosystem.  After all, there are already 7.5 billion of us.  We have huge, possibly insurmountable problems to solve, which are only exacerbated if the population continues to grow.  So reducing global population growth should be on the table as a  topic of public discussion, right next to all the other problems that need to be solved. 

But it isn’t.  Outside of academic circles and activist blogs (both pro and con), the population problem is hardly ever mentioned.  It’s not in the mainstream media or in politics. And that brings me to the third part of my title.  As we face elections here in the U.S., or as some of you face them elsewhere, we must choose candidates who respect science, who are aware of the impact of population growth on world justice and on our planetary ecosystem, and who are willing to study and discuss these issues seriously.  They must also take the search for sustainable economics seriously.  Those who deny the existence of climate change and rising sea levels, who want to do away with environmental protection agencies, who oppose treaties on clean air and reduction of carbon emissions, and who want to open up national parks and other wild lands to mining and other economic exploitation, just don't get it, and must be rejected.