Monday, September 25, 2023

Cactus? Look again!

Aloe erinacea superficially resembles a 
cactus, but closer examination reveals 
that the plant consists of closely-spaced,
spirally-arranged succulent leaves with
spines along the edges. In cacti, leaves 
are done away with altogether, or adapted
as spines.
 The picture at the right is a member of the genus Aloe. We all know the most common member of this genus, Aloe vera,  grown as a garden ornamental, as a source of skin ointment, or for its edible leaves. The species pictured, Aloe erinacea, with its compact, rounded overall shape and prominent spines, superficially resembles a small barrel cactus. Aloes and cacti are both succulent plants adapted to survive arid conditions by storing water in their tissues. In cacti, it is the stems that are modified to store water, while in aloes, it is their leaves. A careful look at this plant reveals that it does in fact consist of a compact series of spirally arranged leaves.  

When unrelated organisms come to resemble each other, it is an example of convergent evolution - similarity due to common adaptation, but from very different ancestors.

I've spoken of this a number of times on this site, as succulents in general are a spectacular example of convergent evolution and the all-important process of adaptation. I've been retired for a number of years, but if were to go back into the biology classroom again today, I would walk up to the front and write ADAPTATION on the board (or powerpoint screen!). I would then proceed to show how everything we see in organisms is a result of this process, which unites genetics, ecology, evolution, and systematics.

Cacti and aloes have very different evolutionary histories leading to their convergence. In this post, I want to emphasize the historical dimension of adaptation. The current functional features of a plant have histories of gradual change, sometimes adding or improving functionality, sometimes changing the function altogether. Leaves themselves underwent extensive series of adaptations in early plants just to become the flat, efficient light-gathering antennae that we know today, and similar series of adaptations happened multiple times in different groups of plants. the very leafiness of leaves itself is convergent in true mosses, clubmosses, ferns, different groups of seed plants.

 In various lineages of plants, leaves were further modified into the parts of gymnosperm cones, the parts of the flower and other reproductive structures. Leaves have also been modified through adaptation into grasping tendrils, sticky insect-catching traps, and other specialized structures. In cacti, leaves disappeared, or were converted into spines, as the stems adapted simultaneously for photosynthesis and water-storage. In aloes, leaves retained their photosynthetic function, while adding water storage.

Both cacti and aloes thus came from "normal" plants adapted for less arid conditions. Among the nearest non-succulent relatives of cacti are carnations, and of aloes they are daylilies, asparagus, and amaryllis. Cacti are eudicots, which typically have prominent stem systems and relatively small leaves. Aloes are monocots, which typically have condensed, inconspicuous, and mostly underground stems, but prominent, elongate leaves. It was "easier" for cacti to adapt their already exposed stems for water storage, but for aloes it was easier to add that function to their leaves, than to redesign their underground stems.  So in adapting to new conditions, organisms modify what they already have, in the simplest way ("along the lines of least resistance").

So in considering the process of adaptation, we must keep in mind that organisms adapt to new or changing conditions by modifying pre-existing structures. Developing new organs from scratch happens rarely, if ever. In plants, stems and leaves have been the most plastic of organs, forming a wide variety of distinctive adaptive organs.

For some zoological examples, take the wings of birds. These highly specialized flight organs evolved from the front legs of their non-flying ancestors, radically changing their function.  It happened separately in flying pterosaurs and in bats, using arm and hand bones differently. It did not end there with the birds, for wings went through another transformation in penguins, from flying organs to swimming organs. In snakes,  legs disappeared altogether or remained as a set of tiny useless bones buried within their muscles, what we refer to as vestigial organs. The same thing happened in whales, where the front legs were modified into flippers, while the hind legs were reduced to buried vestigial structures. So one possible endpoint for a history of adaptations is to disappear!

Studying the history of adaptations in particular animals, or particular organs, is one of the most fascinating areas of biology, helping us understand the strange bedfellows resulting in modern classification (carnations and cacti, asparagus and aloes!), as well as the process of adaptation and the ecology of organisms. Proposed evolutionary scenarios must always include a plausible evolutionary (i.e. adaptive) history of how they came to be. 

The aloes and some closely related genera, incidentally provide another opportunity for a theme and variations expedition like I did with the Amaryllis family a little while ago. These leafy succulents are native primarily to Africa, and here are some photos from my collection:

Close-up of an unidentified Aloe
showing the emergence of new
leaves in the center.

Aloe (or Kumara) plicatilis is
unusual in having its leaves
arranged in a single plane.

Aloe (or Gonialoe) variegata

Aloe pictifolia

Aloe dichotoma is a rare example of a monocot that becomes a tree 
through an unusual type of secondary growth.

Monday, April 3, 2023

The difference between blackberries and mulberries and why it matters

Blackberries grow on prickly vines or brambles, 
and are members of the Rose Family (Rosaceae).
 As I was picking mulberries from a tree in my back yard the other day, I was reminded of the similarity between blackberries and mulberries. They are strikingly similar in appearance. 

Like most dark fruits, they are both rich in nutrients and protective phytochemicals. For the consumer, the primary differences are the somewhat milder, less sweet flavor, and the annoying little green stems of of mulberries. Depending on the climate, one or the other may be easier to grow, and fresh blackberries are generally more widely available in stores. Dried mulberries, however, are becoming increasingly popular and more available. Beyond all that, it's a matter of taste.

Mulberries grow on trees, and are 
members of the Mulberry Family

In terms of the teaching of botany and evolution, however, the similarities and differences between the two berries tell a powerful story. Though they function the same way in natural fruit dispersal, they are not related at all. Blackberries are members of the the very fruitful family, Rosaceae, which includes raspberries, strawberries, plums, cherries, peaches, apricots, apples, pears, rose hips and many more.  Mulberries, on the other hand are members of the Moraceae, which includes figs, breadfruit, and rubber trees. 

The structures of the two fruits are quite different. Blackberries are aggregate fruits, which means that the cluster of drupelets derive from a cone of separate carpels belonging to a single flower. Mulberries on the other hand are technically infructescences, or compound fruits, similar to pineapples. Each drupelet forms from its own tiny flower. So there are fundamental developmental differences that lead to the similar looking fruits.

This might seem like a geeky bit of botanical trivia that would quickly make dinner guests fall asleep, but in the classroom, however, it illustrates some of the most central phenomena of evolution: adaptation, adaptive radiation, and convergent evolution.

These fruits, first of all, are adapted for dispersal by animals, primarily birds, though I have had to keep an eye out for hungry black bears as well while picking berries along roadsides in Washington State. Both go through green and red phases before turning black at ripening. This primes the animals for the coming feast. The berries are sweet, juicy, and flavorful. The animals gobble down the fruits, and the digestive process strips away the juicy tissues, leaving the tiny seeds to pass through the alimentary canal. The animals tend to move about after feeding, leaving seeds in their feces. (See also "What is an Adaptation?)

The different kinds of fruit to be found in the Rose Family are an example of adaptive radiation - the evolution  of a variety of descendant species from a common ancestorAs the descendants of the common ancestor  began spreading into new habitats and new geographic areas, they adapted to local conditions, including local fruit dispersers. 

As other families went through their own adaptive radiations, some descendants encountered the same opportunity for dispersal, and developed similar physical characteristics, but with tell-tale differences in underlying structure. This is convergent evolution - the development of very similar adaptations from unrelated ancestors. The ancestors of the Rose Family happened to have flowers with multiple separate carpels, and so easily evolved into aggregate fruits, while the ancestors of the Mulberry Family had tiny flowers with just one carpel in each, so a similar fruit was most easily developed by grouping the fruits of many flowers together.  I have posted earlier about evolution of cactus-like members in  unrelated families, along with numerous examples of convergent evolution in animals. (See "Of cacti and humans – are certain life forms inevitable?"

Sunday, December 25, 2022

Why do coconut palms lean?

Coconut palms commonly grow along tropical coastlines
in a zone of salt-tolerant vegetation, but not directly in
saltwater. Coconuts may fall onto the beach and be carried
away by high tides, but not usually directly into the water.
 Coconut palms have a distinctive, arching growth form, which is somewhat unusual among palms. Most solitary, tree-like palms grow straight upward rather rigidly. The reason for the coconut palm's graceful arch has led to much speculation online, some of it rather goofy, such as that they lean out over the shoreline in order to drop their coconuts into the water for dispersal. Slightly more plausible is that they lean toward the light, or that they are bent by the coastal breezes. 

While these factors may contribute somewhat to the ultimate shape of the mature palms, I'd like to point to a more fundamental factor: the phase of development that all palms go through after germination called establishment growth.  This is something peculiar to tree-like monocots, which have neither a taproot system nor layered secondary growth. In dicotyledonous trees, stem thickness increases gradually throughout the plant, and the root system branches to keep up with it. (See The Root of the Root Problem)

While coconut palms may appear to all lean toward
the ocean (to the left in this picture), they in fact may lean
inland as well, at least at the beginning. Only a few
at the far upper left of this photo are actually leaning
toward the ocean. Note that the bases of the stems emerge from
the ground at a distinct angle. This is the result of the
early phase of horizontal establishment  growth. 

Most monocots keep their main stems underground as rhizomes, corms, or bulbs, and produce adventitious roots. Leafy shoots and/or flower stalks typically arise directly from these underground stems, and die back after their reproductive cycle. Becoming trees, as in palms, screwpines (Pandanus) or traveler's "palms"  (Ravenala), was an evolutionary afterthought, for which new ways to develop trunk thickness and a sufficient root base had to be invented. (See also The Invention and Reinvention of Trees.)

Monocot trees do this by developing their full stem thickness, along with a mass of permanent adventitious roots, at, below, or close to the ground before beginning their vertical growth. The trunk base can widen only by extending more roots into the soil. This is what we call establishment growth. 

The underground stem of a cabbage
during establishment growth 
is shaped 
roughly like a saxophone, with 
the mouthpiece representing the seed,
and the 
opening of the bell
representing the ever-widening shoot
apex. You have to imagine 
sprouting along the body of the
and leaves emerging from the
open end of the bell.
Drawing from
There are several ways to do this. In cabbage palms (Sabal spp.), for example, the shoot apex first grows downward into the soil, sending up its juvenile leaves  and sprouting adventitious roots as it goes. The stem tip gradually widens and then turns upward. The overall shape of the stem at this stage resembles a saxophone. By the time the shoot apex (stem tip) reaches the soil line, it is as wide as it is going to get, and begins forming a an upright trunk. This takes some 25 years for a Sabal palm.

The production of s series of aerial stilt
roots allows this palm to increase the
thickness of its stem while growing upward.
Other palms, as well as screw pines, begin growing upward immediately out of the seed, as very slender stems that widen as they grow upwards and produce adventitious roots that remain for the life of the plant in the form of stilt roots

The horizontal establishment growth of the coconut
palm stem will proceed to the right in this example.
Photo by Vencel, CC attribution 3.0. 

It appears that the coconut palm follows a third model by establishing its basal thickness along with  a mass of adventitious roots, through a period of condensed horizontal growth, with the lower side of the trunk remaining in contact with the soil. Once it achieves full thickness, the trunk gradually curves upward to achieve a more-or-less upright growth, though it often continues to lean. Since a coconut seedling sprouts out of one end of the coconut, the direction of the horizontal growth phase and the eventual upward curve, will depend on which way the coconut is facing when it sprouts - not so much for any functional reason. 

This is my hypothesis anyway. Those of you who have grown coconut palms from seed can perhaps verify or correct it. 

Monday, November 21, 2022

The major breakthroughs of plant evolution

 As plant life evolved, several major breakthroughs allowed them to greatly expand their footprint across the globe. These breakthroughs were major macroevolutionary shifts brought about by a series of small microevolutionary adaptations. The essential characteristics of Plants are each associated with one or more of these major breakthroughs. Such events are described in more detail in Plant Life: a Brief History, I present here a brief synopsis of those major events:

The earliest known fossil cyanobacteria
formed layered colonies that slowly
built pillar-like formations called
 stromatolites, like these from
present-day Australia. Photo by Paul
Harrison, CC BY-SA 3.0
1. Origin of photosynthesi- this central plant process not only marked the beginning of plant life, but also opened up a vast new energy supply to all life on earth and providing the oxygen supply that allowed for complex food webs and distinctive ecosystems. Though seemingly a long, complex process, different parts of  photosynthesis  evolved separately in more ancient bacteria and were brought together through horizontal gene transferCarbon-fixation or the Calvin Cycle, had its roots in earlier chemoautotrophic organisms, where it was driven, not by sunlight, but by energy captured from sulfur and other compounds bubbling up from undersea volcanic vents. The ability to capture sunlight evolved among other bacteria, likely producing only ATP as its product. When coupled with the carbon-fixation process ,simple forms of photosynthesis came into being. The first organisms capable of modern photosynthesis, which releases oxygen as a byproduct, were the Cyanobacteria, which are still abundant today. Solid evidence of their existence goes back nearly 3 billion years, but they may have been present even earlier. (The first plants) (Cyanobacteria - the super heroes of evolution)

Chlamydomonas, a single-
celled alga CC by-SA 2.0

3. Origin of eukaryotic algae -  Primitive animal-like cells, already equipped with mitochondria, captured cyanobacteria through endosymbiosis, which were "domesticated" to become chloroplasts. (Plants and animals and kleptoplasts - oh my!) This occurred a number of times, resulting in multiple unrelated organisms called algae,  which at first floated as part of the phytoplankton of the seas. Sexual reproduction via cells specialized as sperm and egg evolved in these early algae, along with mitosis and meiosis.

Freshwater charophytes
are related to land plants

4. Origin of multicellular plants - With cells remaining attached to one another, and usually also anchored to rocks and other substrates, multicellular algae were able to branch into extensive light-gathering antenna systems, resulting in the various kinds of seaweeds and freshwater plants like charophytes.

Mosses were among the
earliest land plants, and
continue to thrive in moist
habitats. Modern Sphagnum
mosses pictured here form
vast peat bogs, particularly 
in boreal regions.

5. Invasion of the land
- Green algae adapted already to freshwater habitats, colonized the land, becoming the ancestors of both bryophytes (mosses, liverworts, and hornworts) and tracheophytes  (vascular plants like ferns, gymnosperms, angiosperms). Early land plants survived by developing water-retaining outer layers and internal systems for storing and transporting water. While such plants remained close to bodies of water at first, they created the vegetation that supported the first animal life to leave the water. The hydrostatic, or turgor, pressure within terrestrial plant cells maintains cell and tissue rigidity and drives cell expansion. It also drives the transport of food-laden fluid in the phloem tissue water and, in combination with evaporation and transpiration, helps drive the movement of water from the roots to the leafy plant tops, even in trees 100 meters tall. (How does water get to the top of a redwood tree?) Turgor pressure is also the basis of plant movements, such as the closing of of leaf traps in the Venus fly trap.  (How plants do everything without moving a muscle?)

Ferns produce wind-dispersed spores
that sprout into gametophytes.
6. Invention of wind-dispersed spores.  In the earliest land plants, and still in modern bryophytes and seedless vascular plants like ferns, sexual reproduction was essentially unchanged from what it was in aquatic algae. Sperm cells had to swim to eggs through water-filled channels and films in the soil. Since the distance sperm cells could travel was very limited, early plants produced dormant, wind-dispersed spores through meiosis from diploid sporophyte plants that developed from fertilized eggs. Spores could carry genetic information between populations, thus promoting genetic diversity and greater adaptability.  Spores germinated into haploid gametophyte plants that produced another round of sperm and egg.(The truth about sex in plants

Ovules contain the stages of
from spore,
to gametophyte, to embryo

surrounded by food (seed). 
In this cycad, ovules are
borne on modified leaf-like

6. Evolution of the seed - The seed, called in its early development an ovule, is both a chamber for internal sexual reproduction and a vehicle for the dispersal of the embryo once it matures. Eggs are produced by highly reduced gametophyte plants within the ovules, while sperm cells are produced by even smaller gametophyte plants within specialized spores called pollen grains, which are brought to the ovule by wind or insects.  This liberates plants from the need for water-transmission of sperm to egg, enabling them to live and reproduce in drier habitats, the same way as internal fertilization and laying of desiccation-resistant eggs allowed reptiles to spread through dry terrestrial habitats. The earliest seed plants, took the form of seed ferns, in which pollen-producing sporangia (pollen sacs) and ovules were borne directly on large, fern-like leaves. In more advanced gymnosperms, pollen and ovule forming leaves became distinct from the vegetative leaves and took different forms, most often scale-like structures grouped into strobili (cones if rigid, catkins if soft and flexible). Among living gymnosperms, only some cycads have ovule-bearing structures that still resemble leaves. 

Magnolia flowers have numerous distinct
carpels (uppermost), numerous stamens, all
subtended by a number of tepals (petals/sepals)

7. Origin of the flower -  flowers evolved as a means of manipulating insects and other animals for transfer of pollen from one plant to another (pollination). The seemingly endless  diversity of flowers is reflected in an equal diversity of  insects, birds, and other animals adapted to recognize and feed in  flowers with specific combinations of shape, color, fragrance, and nectar production. Flowering plants, or angiosperms, evolved from ancient seed ferns in parallel with the various groups of modern gymnosperms. Their pollen-bearing structures (stamens) and ovule-bearing structures (carpels), are surrounded by leaf-like petals and sepals, and arranged in a distinctive order in each flower. Carpels mature as fruits that aid in the dispersal of seeds. (What's so primitive about Amborella?)

Grasses dominate extensive areas with
alternating wet and dry seasons. Photo by V. S.
Dustin CC BY SA-3.0

8. Re-evolution of the herbaceous habit -  While ferns and other non-seed-bearing plants were herbaceous, early seed plants and all gymnosperms are woody, as required for the slow development and maturation of their seeds. Angiosperms further shortened the reproductive cycle, so as to make quick-growing, winter-dormant herbs possible again. The most significant group of angiosperm herbs are the monocots, with grasses being the most widespread and ecologically significant herbs on the planet. (How the grass leaf got its stripes) (The grasses that would be trees) Grasses support vast food webs on seasonally dry savannas, and their seeds provide the major source of sustenance for humans around the globe. 

9. Evolution of varied secondary plant compounds - All through the evolution of plants, which are both nutritious and immobile, animals evolved to feed upon them. While some plants, like many algae and grasses, could multiply fast enough to overcome such predation, many other plants have evolved  deterrents, including hard fibers and various forms of spines, thorns, etc., but most importantly toxic or repellant chemicals. Plant chemicals that deter animal herbivores have become numerous and diverse as different species of animals developed immunity to some but not all. By nature, secondary plant compounds are physiologically active and while poisonous in some circumstances, often have valuable medicinal effects. As such, they have been vital to the survival of the human species. (Medicinal plants in our own backyard)

Wednesday, December 9, 2020

A perfect storm of weeds

 A weed is sometimes defined as a plant out of place - or more often an overwhelming mass of plants popping up where we don't want them. It's a definition based on our futile attempts to to remake a landscape into something a human vision of tidiness. To be fair weeds are often exotic plants - invasive species from another continent freed from their usual constraints of competitors and predators. And so, weeds are also bad for our natural ecosystems, not just to our landscaping vanity.

Weeds are mostly plants that are really good at spreading into disturbed habitats. They multiply rapidly, often asexually, and fill vacant ground or the exposed sides of forests. They are a vital part of succession, preserving soil, nutrients and moisture. And so such plants are good for their native ecosystems. Our native grape vines and blackberries, however, can also become a nuisance at the edge of woods, sometimes creeping into yards, and so give the landscaper an ethical dilemma.

A native species of morning glory begins to reclaim an abandoned logging road in Papua New Guinea.

Up north, fastidious weed-haters spend hours in the 

Syngonium (Nepthytis) podophyllum is valued as
a house plant, but it can escape into native woods.
Here it clambers into a conservation area near a
housing development in Florida.

spring and summer pulling up dandelions one-by-one from their lawns, only to have them repopulate the next season from the one that got away, its seeded parachutes having been blown across the yard by a visiting grandchild.

While you in the north can relax during the winter, we in Florida, continue to battle with the "Vines from Hell" that never take a rest. Our nastiest weeds are climbers and creepers that can smother a bush within months, or just as easily march through beds and across lawns. They are vines that not only grow upward, but also on the ground, sprouting roots as they go, and this is where they are most troublesome. A simple vine can be severed at the base and pulled from the trees, but removing the rooted bits of one of these creepers from the soil is a nightmare. We chop them up, pull them up, dig them up, but if we leave one tiny fragment, it will come to life again like the splinters of the broomsticks in the Sorcerer's Apprentice. All it takes is a single node, with a single tiny bud.

Missed bits of Syngonium resprout in an area that
was recently cleared of it.

So in our Florida yards some of our worst weed nightmares are Nepthytis (Syngonium podophyllum), flame vine Pyrostegia venusta), air potato vine (Dioscorea bulbifera), and skunkvine (or stinkvine), (Pedaria foetida). Skunkvine comes from tropical Asia and air potato from tropical  Asia and Africa. Flame vine and Nepthytis come from tropical America. 

The air potato is a member of the true Yam Family (Dioscoreaceae), while sweet potatoes, which are sometimes called yams, are in the Morning Glory Family (Convulvulaceae). Real potatoes are in the Tomato Family (Solanaceae).  Air potatoes can evidently now be controlled by a beetle from Nepal, and so is not seen in Florida quite as much.

There are a couple more that don't climb trees, but are even more adept at creeping horizontally through beds and lawns. The Boston fern (Nephrolepis exaltata), a popular house plant, will escape into moist woods and form dense colonies in Florida. It is a native of tropical regions around the world, and will actually freeze to death if it attempts to escape anywhere near Boston! Marsh pennywort (Hydrocotyle vulgaris), native to Europe and North Africa, can choke out lawn grass, particularly in moist areas near ponds.

One of the most rampant vining weeds in central Florida is Skunkvine, here twining its way through a Ligustrum hedge.

Flame vine is an attractive ornamental vine, 
but it can smother trees and also spread across
the ground, rooting as it goes.


Boston fern seems to be an innocuous house or bedding
plant, but can form thickets in moist woodlands when it
escapes. It extends horizontally through the soil by
slender runners that sprout new plants every few feet.


The flowers of pennywort are in umbels, demonstrating its 
relationship to members of the Carrot Family (Apiaceae or 
Araliaceae). Seed production is not important for local
spreading, but is likely responsible for long-distance dispersal.

Air potato vine is a member of the true Yam
Family (Dioscoreaceae). It clambers into

Air potato vines produce small tubers on their
stems, which fall to the ground and start new vines.
Caution - they are not edible. Photo by Karen Brown;
University of Florida; posted on USDA National
Invasive Species Information Center. 

Tuesday, August 4, 2020

Grasping at Straws

Vining plants have an amazing ability to grab onto a  trellis, fence, or a twig on another plant by curling around it. It's an adaptation that allows the vine to grow rapidly upward using other objects for support. This gives them a distinct advantage over tree or shrub saplings that need to build their own woody support as they grow upwards. But how does it work? 

The process is called thigmotropism, or touch-induced growth response.  Specialized organs called tendrils, or sometimes the stem of a young plant itself, can sense contact with a nearby object and alter their growth pattern so as to bend toward it.  If the object is rigid enough and not too thick, the tendril or stem will continue to bend and coil around it. 

The tendrils of a bitter melon vine stretch out ahead of the shoot apex.

When a tendril encounters an object, such as this actual straw 
recruited for the demonstration, it will grasp it by wrapping
around it.
The tendril of a passion fruit vine seems to have tied itself into some kind of nautical knot 
to secure its support on a fence.

 Thigmotropism is similar to phototropism and gravitropism, which are the bending responses to light and gravity respectively.  

In the light response, light-sensitive pigments create an inhibition of  the growth hormone, auxin, on the lit side and then the opposite side grows faster, bending the stem toward the light. 

Gravitropism comes into play underground, causing roots to grow downward and buried shoots to grow upward. For example, if a root emerges from a sprouting seed sideways, tiny crystals in the cells of the root tip, called statoliths, settle downward to the lower surface of the root, causing the upper side of the root to grow faster and bending the tip downward.

The mechanism in thigmotropism is not as clear and not always the same. Since thigmotropism occurs in different kinds of organs in different plants, it has certainly evolved independently many times.  For example, just within the Legume Family, peas have evolved to climb by tendrils, while  beans climb by twining their stems around a support.

 In general though, the touch of an object deforms the surface of the epidermal cells, and growth closest to the object is suppressed. Continued growth on the opposite side causes the stem or tendril to coil around the support. 

In the noxious weed, skunk vine, the stems themselves wrap around
the support. Bean plants twine in the same way.

Tendrils may be separate organs, or in the case of this climbing lily,
Gloriosa, just the tip of the leaf. Photo by SAPlants, posted on Wikipedia,

The genus Clematis is unique in the Buttercup Family,  Ranunculaceae,
in its vining habit. Its young compound leaves are thigmotropic and can
wrap around slender objects.
Some climbing plants use a completely different means of 
attaching to a support. The most unusual I've ever seen is this
climbing Sundew from southwestern Australia, which re-purposes
some of its sticky insect-catching leaves for attachment.

Sunday, July 5, 2020

The folded leaves of Iris

In this Bearded Iris  the leaves are folded and flattened,
forming a fan perpendicular to the tip of the rhizome.
Many members of the Iris Family exhibit a peculiar, fan-shaped arrangement of their leaves. Leaves that are lined up on two sides of the stem in a single plane are called 2-ranked, or equitant.  Such an arrangement of leaves is not uncommon, occurring in the Traveler's Palm, Ravenala madagascariensis, for example.

In the Traveler's Palm, leaves are equitant, but have
conventional, spreading blades, with exposed upper and 
lower surfaces.
The leaves of the Iris connect to the rhizome in a circle, as
in most monocots, but above that,the the two
sides fold together tightly forming a narrow channel
through which newer leaves emerge. Still higher, the two
sides  of  the leaf become completely joined together, forming
what  appears to be a simple, sword-shaped leaf blade.

The bud of a new inflorescence pushes up through the
center of the fan.
But what's most interesting in the Iris is that the leaves are folded, with the two sides fused together into a seemingly simple structure.  It's as if someone has taken a hot iron and pressed the whole clump of leaves into a flat sheet in preparation for mounting in a herbarium. You can see such leaves in many members of Iris, Gladiolus, and related genera. It has evolved independently in unrelated monocots such as Acorus (Acoraceae) and Lachnanthes (Haemodoraceae).

Such folded leaves are called unifacial (one-faced), because both sides are actually the same side - technically the abaxial side. The upper, or adaxial side of the leaf is totally internalized.

You can see the folding most obviously at the bases of the leaves where the two sides remain separate to form a leaf sheath. New leaves emerge from the center of the fan through the folded bases.
The inflorescence results from the elongation
of the rhizome tip, with long internodes
between leaves that are reduced in size. Each
leaf is open at the base, but fused into a
solid upper portion.
Like the leaves in the main fan, those on the inflorescence
stalk are open at the base, but fused together in the upper part.
Ultimately, the spectacular flowers of the Bearded Iris open, beginning at the top. Other flowers
will emerge from the bracts lower down. Incidentally, this is a rare sight in central Florida, where these pictures were taken. Only recently have "reblooming" varieties of the Bearded Iris been grown successfully here.