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.

T

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
trees.

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. 

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,
CC-BY-SA-4.0.

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.

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.

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
leaves.

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.

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
year-round.

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).

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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.