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.

Sunday, May 3, 2020

The Leafy Origins of Sepals

The sepals of a rose bud are green and photosynthetic
like fully developed leaves, and like the one on the left,
sometimes even appear to partially subdivided like full
From an evolutionary point-of-view, it is generally accepted that the parts of the flower originated as modified leaves. Though there is controversy about the nature of the earliest carpels and stamens, the leaf-like nature of petals and sepals is abundantly evident. Sepals are generally the most leaf-like, no doubt because they are the most recently evolved of the flower parts, and may have originated separately in different lines of early angiosperms. 

In some archaic flowering plants, such as Magnolias, petals and sepals intergrade, with no clear distinction between the two, and they are called tepals.

In Magnolia, there is a series of similar outer, leaf-like
structures that are colored like petals, though the outer ones
serve to protect the bud during development, as specialized
sepals do. Photo from Wikiwand, License: Creative
Commons Attribution-Share Alike 3.0
Upon looking at a rosebud, the leaf-like nature of the sepal is evident. The occasional sepal that takes on even more of the subdivided shape of the full leaf, emphasizes the point.  Leaves, bracts, sepals, and other flower parts develop from outgrowths of the apical meristem, and so in their earliest stages look the same. As each develops for its specific function, specialized genes kick in to determine the final shape, color, and other physical features of the organ. The fact that some rose sepals look a little more like normal leaves than others shows that the genes for full leaf development that are normally suppressed, can sometimes be partially expressed.
In Clusia, the leaves are opposite with each pair at right angles to the
preceding pair. A series of small bracts and two pairs of colored sepals
continue the pattern. it's truly hard to see where bracts end and
sepals begin.

The development of similar, but modified, organs, from outgrowths of the apical meristem, is called serial homology. Homology in general refers to different body organs that were the same in ancestral species, but have become specialized for different functions in  more specialized species.  The classic examples in animals are front legs that have become specialized as wings in both birds and bats, and arms with grasping hands in primates.  Serial homology can be seen in animals with segmented bodies. Insects and crustaceans, for example, descended from many-legged, centipede-like ancestors, but now have specialized walking legs, reproductive organs, claws, mouth parts, and other specialized organs, all representing modified legs.
Lilies, like many monocots, have what appear to be six petals,
but three of them are actually petal-like sepals
Serial homology of leaf-like organs in plants suggests that at one time there were only leaves, as in early seed ferns, that did everything: photosynthesis as well as bearing pollen sacs and ovules, and all had the same shape.  As plants progressed, a division of labor came into being, with some leaves continuing the primary photosynthetic function, while others became specialized as bracts or floral organs.  In previous posts I have described even more bizarre leaf modifications, such as insect-catching traps.

Trilliums are monocots only distantly related to the true lilies,
and display three leaf-like sepals, most likely as did the
ancestral monocots.
The ancestral set of genes that orchestrated the
development of leaves was supplemented with new sets of genes that served to modify the embryonic leaves for specialized functions.  The new sets of genes both suppressed the full development of the original leaf size and shape and directed the development of specialized features. Serial homology along a single shoot, from leaves to bracts to flower parts, shows that these different sets of genes are turned on and off in an orderly way.

Sunday, March 22, 2020

Why are Anthuriums red?

One of my favorite plants is this cultivar of
Anthurium andreanum, with spathes of
pure, bright red. If treated well, it will bloom

More correctly, the title of this post should read "why are the spathes of some species of Anthurium red?' - but that's way too wordy for a title.  The fact of the matter is that there are some 1000 species of the genus Anthurium, and only a few species have red spathes.

The most commonly cultivated species is Anthurium andreanum, available in many different cultivars and hybrids. It is native to Ecuador and neighboring Columbia. Little is known about the species reproductive biology in the wild, but the bright red spathes literally scream "birds!" Well, not quite literally, but bright red colors in plants usually are an adaptation for attracting birds, either for pollination or fruit dispersal.

It has been speculated that the red to orange spathes in wild plants help birds find the ripe fruits, which they would eat, fly off, and thereby disperse the seeds. It's a common dispersal adaptation, found even in the most archaic of angiosperms (e.g. Amborella), and it may very well be true in this species, as well as many other species of Anthurium.

In all members of the Aroid family, flowers are tiny and crowded onto the elongate spadix.  There have been many observations of pollination by tiny flies, beetles and other insects in various species of Anthurium, and it has been assumed that birds would take no notice of them. That was until recently.

A 2019 article by Bleiweiss et al. provides the best evidence so far for bird-pollination in Anthuriums with red or other brightly colored spathes. It wasn't the first evidence of the possibility, as Bleiweiss cites a paper from some 20 years earlier by Kraemer and Schmitt making similar, if not as thorough, observations.

This reminded me of seeing nectar drops on an Anthurium andreanum specimen in the Bailey Hortorium greenhouse at Cornell, some 50 years ago, and wondering the same thing.  That picture is posted below. You can see the nectar exuding from several of the tiny flowers.  A patient hummingbird could get a decent meal by collecting a series of these droplets.

Makes me think about some other pollination mysteries ... stay tuned.
Anthurium andreanum growing in a greenhouse at Cornell University around 1970. note the tiny droplets on some of the upper flowers (enlarged below).

Friday, March 13, 2020

Plant wrappers - leaf sheaths and bracts

While the young leaves of Magnolia
are developing,  they are each wrapped in a white
bract (technically a specialized, bract-like stipule).
Leaves are the most plastic of all plant organs.  That means that they can be modified in endless ways
to result in a mind-boggling variety of shapes. Through evolution via adaptive modification, leaves form an endless array of light-gathering antennas, from the giant fronds of palms to the tiny scales of a juniper twig, but beyond that, have adapted into tendrils, insect-catching traps, and even the parts of the flower.

In the fennel plant, the broad basal portions
of the leaves, the leaf sheaths, overlap to protect
the developing shoot apex.
Today, I'm talking about leaves, or parts of leaves, that form wrappers around tender growing parts of the shoot.  Modified leaves that do so are called bracts, and the modified lower parts of leaves that do so are called leaf sheaths.

A bract is a whole leaf, though it is typically smaller than a regular leaf, simpler in shape, and often colored differently. In some cases, brightly colored bracts serve as part of the apparatus for attracting pollinators, and may even appear to be petals.

A leaf sheath, on the other hand, is the broad basal part of typically large, complex leaves that surrounds the growing tip of the shoot. The rest of the leaf - typically a petiole and a blade - is typically full-sized,

As flowers and leaves emerge from a Crocus corm
in early spring, they are protected by white bracts.

In this bromeliad, Tillandsia cyanea, a fan of colorful
bracts help keep the plant on the radar of pollinators
as the flowers emerge one at a time.

Pachystachys lutea, or yellow shrimp plant, forms
a cone of yellow bracts to attract pollinators to
the white flowers.

As for leaf sheathes, some of the most spectacular are found in palms, but virtually all monocots form a leaf sheath when young.  Leaf sheathes attach to the stem in a complete circle when young, but typically splits apart on one side as the leaf matures and the stem within it expands.  In others, such as the royal palm, the overlapping leaf sheaths of the functioning leaves remain as a smooth, tight, crownshaft.

The leaf sheathes of the royal palms (Roystonea spp.) can be more than  four feet long.  They remain intact as complete
cylinders, forming what is called a crownshaft.  Photo from Palmpedia, photographer not indicated.

The "trunks" of banana plants are made up entirely of leaf sheaths, that may be more than three meters long, wrapped around each other (see "The invention and reinvention of trees")
As each new leaf emerges from the tip of the shoot
of a banana plant, its sheath is longer than the previous
ones.  This builds up a pseudostem of overlapping,
cylindrical  leaf sheaths.

Recall from "The underground plant movement" that the bulb of an onion or amaryllis is also made up of leaf sheaths that fill up with food and water, and are left as storage organs as the leaf blade on top of them dries up and disappears.
In a young onion plant, the leaf sheathes just above
the roots begin to fill with food.

When the onion plant goes dormant for the
season, the food-filled leaf sheathes remain,
forming  the rings of the onion.  The
outermost sheaths dry out to form a
protective tunica.
In many irises, gladioli and other members of the
Iridaceae, the leaf sheath is folded and the entire
shoot looks like it has been pressed with a hot iron.
Note that the newer leaves emerge from the
overlapping, folded leaf sheathes.

Friday, December 6, 2019

Good fire, bad fire

sometimes seems that all wildfires are bad.  Forests burn down, homes and whole towns are destroyed, carbon dioxide is released into the air, valuable wood is destroyed, and wild animals are killed or driven from their habitat..

So it is surprising to hear for the first time that wildfires are natural and necessary in many ecosystems.  They become bad basically only because of our own interference.

As in many pines, the seed cones of  Banksia in
Australia, open only after a fire to release their
Ecosystems in which fires are a normal part of maintenance or renewal are those in which there are distinct wet seasons and dry seasons.  During the wet season, there is abundant growth of trees, shrubs, grasses, and other herbs  During the dry season, leaves and twigs fall from the woody plants and grasses dry out.  Typically, this debris accumulates faster than it can decay, so builds up from year to year.  Sooner or later a lightning strike will ignite the accumulated debris, causing a wildfire.

 Burning removes the debris, releases nutrients back into the soil, clears out the undergrowth, trims dead branches from the trees.  In some cases, shrubs are burnt to the ground, but re-sprout quickly at the beginning of the next wet season.

Plants in these areas are adapted to these periodic fires. Pine trees, for example, survive moderate fires, and require the ground to be cleared for seeds to germinate.  In many species, seed cones will not even open until heated by fire.  Where fires are prevented for a number of years, ground vegetation becomes thick and pines do not reproduce, and when fire inevitably strikes, it is more intense and trees die.  For these reasons, foresters often conduct regular controlled burning to prevent more intense fires later.

In the pine flatwoods of Florida, fires remove the undergrowth and debris, clearing the way for 
germination and growth of pine seedlings.  Without fires, pines would gradually disappear.  
Their thick bark protects the trunks, and the upper branches are spared as well, as long as fires  
are frequent and not too intense. Saw palmettos, Serenoa repens, cover much of the ground  
here, but can be seen here recovering quickly after a fire.  
Bulb plants, like this Florida native Lilium catesbaei,
survive fires below ground.  Plants that sit out
the dry fire season are particularly common in
California, South Africa, and Australia.

The California chaparral and other forms of Mediterranean vegetation are adapted to winter rains and long summer droughts, and are also fire-maintained. It is here where we see shrubs well-adapted for re-sprouting after burning to the ground.  Grasses, and wildflowers also thrive after fires, and are suppressed if the shrubby overgrowth becomes too thick.  Between the pine forests and the chaparral, much of California is thus prone to natural fires, setting up an unfortunate conflict between nature and people building homes on vegetated hillsides.  The same tragic conflict can be seen in many parts of Australia and southern Africa.

Wildflowers, such as this Liatris, flourish where
fires are frequent in Florida.

The only places where wildfires are rare are in areas with reliable, year-round rainfall, or in areas of practically no rainfall.  So rain forests, temperate deciduous forests, and deserts do not normally experience fires.  In the rain forest, and temperate forests with precipitation all year long, vegetation rarely dries out, and debris is decomposed quickly.  In the deserts, vegetation is sparse, and very little debris is produced. It's the areas between these extremes that rely on fires.  Aside from the chaparral and pine forests mentioned above, this would include the grasslands and deciduous tropical forests that cover vast areas of Africa and tropical America.

So how do we humans turn good fires into bad fires?  There are several ways.

First, by overzealous prevention of fires where fires should normally be occurring, we allow more debris to build up, allow opportunistic undergrowth vegetation to run rampant, setting up for a more disastrous fire when lightning eventually strikes.  In these disastrous fires, pines are not only pruned, but burned to the ground.  Such fires may be so hot that even the root systems of normally resilient shrubs are destroyed, and then do not re-sprout.
After the big fire in Yellowstone National Park in 1989, grasses and wildflowers, such as the pink fireweed, Epilobium angustifoliumgrow abundantly, a boon to local herbivores. 

Second, climate change is resulting in the expansion of dry seasons into formerly wet forests in many parts of the world. The intensity of droughts, as well as floods, hurricanes, and blizzards is increasing. This is not currently seen as a significant factor in the catastrophic fires in the Amazon Basin, but are a factor in the desertification of the African savannas.

Third, clearing and burning of forest for conversion to farm or grazing land, which is occurring in the Amazon Basin at an increasing rate, can get out of hand during dry periods and burn more extensive areas than normal. Fires are normal only where rain forest transitions into deciduous tropical forest, mostly along the southern fringe of the Amazon forest. Clearing of the forest, apparently supported by the current government, is also fragmenting the forest, causing it to get drier and less able to sustain itself.

So the burning of the Amazon rain forest, unlike the routine burning of the chaparral and pine forests, is a tragically bad fire.  It is resulting in a significant loss of biodiversity and loss of photosynthetic activity that could help offset climate change.
Rauch steigt in Brasilien auf - die Feuer haben sich in den vergangenen Tagen ausgebreitet
The fires in the Amazon Basin are largely due to human activity.  They are a tragedy because of the huge loss of  biodiversity, release of CO2 into the atmosphere, and loss of photosynthetic oxygen replenishment. 

Wednesday, September 25, 2019

Pitfalls of the long branch

Long branches in phylogenetic trees represent lineages of  organisms that have been around for a long time, but exist today as only one or a few species.  A few years ago, I discussed two examples in detail: the monocot genus, Acorus, and the archaic angiosperm species Amborella trichopoda.  I feel that the topic is worthy of a review, especially for newer readers who may not have gone back to the older posts. 

In both cases, these lineages branched off very early, over 100 million years ago, but have left no fossils, and have no close living relatives.  The Amborella branch is the earliest surviving lineage of angiosperms in general, while the Acorus branch is the earliest surviving lineage of monocots.  Expressed in a different way, Amborella is the sister group to all other angiosperms, and Acorus is the sister group to all other monocots.

At the level of phylogenetic analysis, such long branches have often been problematical, with "long branch attraction" leading occasionally to errors in the resulting phylogenetic tree.  This has been much discussed, and there are ways to correct for it, but this is a very technical issue. If you want to learn more, you might begin with .  Begin with this Wikipedia article, and go from there.

 In both cases, however, many phylogenetic analyses have confirmed the ancient position, and length of these two branches, so that is not a question  here..

Such long branches can lead to errors of interpretation at another level, however. A common misconception is that what we see in the current species, which occupy the very tips of these ancient lineages, will be similar to the  ancestors from which the lineage began, i.e. that these are archaic or primitive species.

But think about it.  These lineages have been around for more than 100 million years  (140 million for Amborella, 120 million  for Acorus).  Isn't it likely that the occupants of these lineages have changed somewhat over all those years?

Amborella fruits are single-seeded drupes, adapted for
dispersal by fruit-eating birds.  This is a specialization
that has evolved many times among angiosperms, including
most famously, cherries. Early angiosperms most
likely had fruits that split open to release several to many
seeds (see Were the first carpels plicate or ascidiate?
Small, unisexual flowers in dense clusters
 is also a specialization. Photo courtesy Joel McNeal.
Modern phylogenetic analyses are based primarily on molecular (DNA) comparisons, so in-and-of themselves tell us nothing about changes in the characteristics of the plants occupying the lineages.  So there is no direct basis for inferring what the first species in a lineage looked like or in  what ways their modern descendants may have changed.

As I argued in the previous posts, both Acorus and Amborella, as they exist today, exhibit a mix of ancient and specialized characteristics. They are both well-adapted to their environments, and have some distinctive specialized characteristics, particularly in their adaptations for pollination and seed dispersal. The Acorus and Amborella lineages have been around for such a long time, that it is rather absurd to think that they have not changed at all during that time. For groups that have good fossil records, we can trace such changes.  Fossils, for example, tell us that we modern humans have changed a great deal from the first members of our genus, even more from the ancestral genus Australopithecus!

The spadix-like inflorescence of Acorus led early
taxonomists to classify this genus with the Aroids.
Since the two families are not closely related, it is likely 
that the similarity is due to convergent evolution, driven by
adaptations for pollination. A spadix is a highly specialized
 way to arrange flowers and has evolved independently in a
number of families, including the Aroid, Palm, and 
Cyclanthus families. It is likely that the early monocots had
looser arrangements of flowers, more like those in most
Alismatales, and that dense flower spikes were not
characteristic of the first members of the lineage.
The folded and fused (equitant) leaves of Acorus, are
also a specialized adaptation that has occurred in many
unrelated families, most famously in several members
of the Iris Family.
How do we know, or at least develop hypotheses, as to what changes have taken place in a lineage in the absence of any fossils?  We can look at the characteristics of other early branches to see what they have in common, and hypothesize that the shared characteristics were present in their common ancestor.

 We can also analyze how particular characteristics might have arisen as adaptations to natural selective pressures, and determine which are most likely ancestral, and which are more specialized. Adaptations arise in logical sequences and often become canalized in non-reversible directions (see What is an adaptation? and G. L. Stebbins and the process of adaptive modification)

Both in comparison with other related groups, and in considering likely sequences of adaptations, Amborella and Acorus are specialized in some ways. For Amborella, small, numerous, unisexual flowers in clusters, and red, single-seeded fruits are both features that are more specialized than in other archaic angiosperms. For Acorus, the dense spikes of flowers with fused carpels (see also Were the first moncots syncarpous?) and the the leaves with the two sides fused together (equitqnt are specialized features, that have evolved independently in a number of families from more generalized types.