Thursday, April 12, 2012

The underground plant movement

Plants in general must bask in the sun day after day to soak up the energy they need for life activities.  In doing so, of course, they face certain hazards.  Being immobile, they are essentially "sitting ducks" for everything from plant-munching animals to fire, drought, and winter freezes.  As discussed earlier in "How the grass leaf got its stripes," plants in general employ one of two basic strategies for coping with such hazards.  Trees and shrubs tough it out with permanent woody shoot systems that resist damage with protective tissues or toxins or by dropping their leaves during the adverse seasons.  Their dormant buds are typically wrapped in tough scales.  They may simply rise above threats from grazing animals or ground fires, as do the Acacias of the tropical savannas.  Their competition with one another for light often results in forests. 

Grasses dominate in areas where moisture is too sparse for forests.  They have plenty of light, and escape most environmental threats by keeping their main stem system and buds below ground.  Like the periscopes of submarines, their leafy shoots and flowerstalks rise above ground to do their business, sending food reserves to storage organs below ground, or drawing upon those reserves to make seeds.  When things get too rough upstairs, these aerial shoots are abandoned, and replaced when better growing conditions return.

Grasses are not the only plants to "go underground."  A great many perennial herbs follow similar strategies, as do the unique class of plants called biennials.  They all achieve similar results, but what differs is the nature of the underground storage structures.  In different groups of plants, roots, stems, and even leaves, are recruited for this job.

The carrot, Daucus carota (Apiaceae) is mostly a thick taproot.
A short stem at the top produces the cluster of leaves.  Photo
by jonathunder, Wikimedia commons.
In most biennial herbs, a specialized taproot, or sometimes a hypocotyl  (to be explained shortly!) swells up with food reserves, resulting in a carrot, radish, beet, parsnip, turnip, or one of many other type of root vegetable.  These are radially symmetrical plants with an axial root system, similar to trees and shrubs.  The expansion of their storage organs in fact is due to the activity of a type of cambium similar to that which produces layers of wood in a tree.  They can be considered highly reduced trees in this respect. 

Biennials store up food reserves during their first season of growth, and use up those reserves making flowers and seeds during the second season.  Then they die.  Of course such swollen taproots are great sources of food for humans and other animals, and we cheat them out of reproducing by harvesting after the first season.

In the radish, Raphanus sativa (Brassicaceae), the
short section of the seedling stem below the cotyledons
(the hypocotyl) swells into a storage organ. From Transeau, et
al., 1940, Fig. 114.
All of the above-mentioned "root vegetables" appear to be actually roots, but some are actually a different, adjacent part of the plant.  When a seedling emerges from a seed, it consists of three sections: the root, the shoot, and something inbetween: the hypocotyl ("below the cotyledons").  The hypocotyl is the usually inconspicuous section of stem between the root proper and the seedling leaves (cotyledons) that mark the beginning of the leafy shoot.  In some biennials, such as radishes, it is this tiny section of stem that swells into the underground storage organ, not the root proper.  This is of utterly no importance to the master chef, but an essential piece of information for the botany-geek-wannabee.

In the sweet potato (Ipomoea batatas),
adventitious roots from the vine swell to
become storage organs. Photo by H. Zell,
Wikimedia commons.
A sweet potato (Ipomoea batatas in the Morning Glory Family, Convulvulaceae) is also a root, but not a taproot as in a carrot. A sweet potato forms from an adventitious root that emerges from the vine creeping along the ground. Similar are the tuberous roots of Dahlias.  Regular ("Irish," "white," "Idaho") potatoes, on the other hand are specialized stems called tubers. The "eyes" on a potato are buds that can sprout into new leafy shoots.
The potato, Solanum tuberosum, is a
swollen underground stem with many
buds ("eyes") that can develop into new
leafy shoots. Photo by Donna, Wikimedia

A ginger rhizome (Zingiber officianale), with a new rhizome
section developing (upper right).
The most widespread kind of underground stem is the rhizome.  This horizontal stem creeps along under the ground, or just above it, branching to form new rhizome sections, and sometimes expanding into an extensive colony.  A ginger or bearded iris is a good example of a rhizome that is also a food storage organ.

A new shoot is developing on top of the corm of this
Amorphophallus titanum. It will form a new corm at its
base, while the old corm withers. Photo by stickpen,
Wikimedia commons.

A more specialized type of underground stem is the corm.  Often confused with a bulb (see below), a corm is a stem filled with solid storage tissue.  Found mostly in the Iris and Aroid Families, but also in water chestnuts (Sedge Family). Corms are short and fat, with a single dominant bud facing directly upward.  As that bud begins growth, the base of the new shoot swells to form a new corm on top of the old one, and sends out new adventitious roots.  The old corm gradually decays, and the newest corm is pulled down to replace it by the roots, which physically contract and shorten (contractile roots).

An onion, Allium cepa, is a bulb
made up of the swollen, concentric leaf
sheaths of regular foliage leaves. Photo
by Amada44, Wikimedia Commons.
Finally, a true bulb is actually a big underground bud, made up of a cluster of modified leaves or leaf bases that swell up with food.  An lily, onion, or amaryllis bulb is a good example.   The bulb of the onion or amaryllis is made of of the concentric fleshy leaf sheaths of ordinary leaves. As each new leaf forms in the center of the bulb, it produces a long photosynthetic blade that emerges from the top, while the cylindrical sheath slowly expands within the bulb.  The collective fleshy sheaths ("onion rings") constitute the dormant bulb when the leaf blades wither at the end of the growing season. The oldest leaf sheaths on the outside of the bulb eventually dry up, becoming thin and paper-like.

In the true lily (Lilium), on the other hand, the bulb is a loose cluster of short, modified leaves, swollen with food and water. Like the vegetative leaves that develop on the elongate aerial shoot, these leaves have a relatively narrow base, rather than a cylindrical leaf sheath.
In the bulbs of Lilium (left) the food-storage organs are short, modified leaves, rather than the
sheaths of vegetative leaves.  From Brown, the Plant Kingdom, 1935.

License for photos from Wikimedia Commons:

Thursday, April 5, 2012

Whatever Became of the Snapdragon Family?

For centuries, botanists have grouped similar plants into the taxonomic category of family.  Plant families are a very important way of recognizing relationship and predicting useful characteristics.  For example, if we find a valuable medicinal property in one member of a family, we are more likely to find similar properties in other members of the same family than in species outside the family.   Recently, I wrote about the important Grass Family, and the similar edible properties of many grain species in the family.  Other familiar families include the Rose (Rosaceae), Tomato (Solanaceae), Legume (Fabaceae), and Sunflower (Asteraceae) Families.

Butter-and-eggs, or Linaria vulgaris, is a
close relative of the snapdragon, and
no longer in the Scrophulariaceae.
The Snapdragon Family (Scrophularaceae, also known as the Figwort Family) used to be a happy but rather large family that included snapdragons, mullein, monkeyflowers, foxglove, veronica, torenia, Indian paintbrush, calceolaria and many other attractive garden and wild flowers.  It is not a major family for medicinal plants, but the foxglove is the source of the important heart stimulant, digitalis. Many other members of the family (e.g. snapdragons themselves) provide ornamental bedding plants worth millions worldwide.

Sadly the snapdragon family has suffered a nasty divorce, and what we now call the Scrophulariaceae no longer contains the snapdragons.  The old Scrophulariaceae has in fact been split into at least six smaller families (Olmstead, et. al 2001). The pieces, it was found, were not closely related to each other, some pieces were actually closer to other families, including the mint, gloxinia, and verbena families.

The family is a victim of the more precise analytic tools of the late 20th century and the new phylogenetic taxonomy. The similarities among the members of the old Scrophulariaceae were superficial, it turns out. The general flower shape and form of the seed capsule evolved many times from different ancestors (convergent evolution) because they were adapting to similar pollinators and seed dispersal strategies. In sum, the old family was polyphyletic

The story involves all the drama and complications of modern plant taxonomy.  For two centuries, botanists have worked to define more "natural" families.  Natural families contain genera that share fundamental characteristics, including similar structural features of flowers and fruits.  In the late 19th century the word natural took on the new meaning of evolutionary relationship.  Following Darwin's lead, biologists interpreted fundamental similarities within families, genera, and other categories as due to common ancestry.  For the next century or so interpretations of natural similarity and evolutionary history went hand in hand.  More and more information from anatomy, chemistry and genetics became available, often confirming, but sometimes toppling earlier assumptions about relationship.  The information available became so huge that putting it altogether into a master classification of plants relied on the experience and gut feelings of a handful of the leading taxonomists.  But the classifications of these wise old men often differed markedly, as much of the information could be interpreted in different ways.

In the middle of the twentieth century, there were many attempts to decipher evolutionary history (or phylogeny) with rigorous and objective statistical tools.  The most successful of these was cladistic analysis, a process in which similarity among a group of organisms was distilled into distinct and narrowly defined characters, and the organisms sorted out based on the number of shared characters.  Organisms with the most shared characters came out close together on a tree-like cladogram.  The branches of the cladogram were then grouped into taxonomic categories, and the cladogram was also interpreted as representing the evolutionary history of the group (i.e. as a phylogenetic tree).  Cladistic analysis,now routinely applied to structural, chemical, and genetic data, remains the core of systematic biology.

This lousewort, Pedicularis bracteosa,
 is now in the Orobanchaceae.
So then, where did the snapdragons end up?  The snapdragon genus (Antirrhinum), along with foxglove (Digitalis), Penstemon, Veronica, and butter-and-eggs (Linaria) ended up fortunately together, in what might now be called the new Snapdragon Family, or technically the Plantaginaceae.  The very similar-looking monkeyflowers (Mimulus) are only distantly related and have been moved to the family Phrymaceae.  Indian paintbrush (Castilleja) and Pedicularis now belong to the Orobanchaceae, which contains mostly parasitic species. Torenia is in the Linderniaceae, and Calceolaria is in the Calceolariaceae.  What remains in the Scrophulariaceae are the figworts (Scrophularia), Mullein (Verbascum) and some other original members, with the addition of  some genera that used to be in other families, including Myoporum and Buddleja

Nothing is more snapdragonish than a
monkeyflower, but this common western
American genus is now in a family
no one has heard of - the Phrymaceae.
Confused?  Don't worry, most professional botanists and horticulturists are as well.  Many websites are still using the name Scrophulariaceae in its original broad sense.  It's a trainwreck out there.

Plantago is a genus of weedy,
wind-pollinated herbs, that is
unfortunately atypical of the
new Snapdragon Family. Photo by Bernd Haynold
The name of the new Snapdragon Family, Plantaginaceae, is a real irritant to many of us old-timers.  It is based on the genus Plantago, a genus of small, inconspicuous, wind-pollinated herbs known as plantains (not the big cooking bananas!).   Because of its highly reduced flowers, its relationship to snapdragons, etc., was not recognized, and it had been in its own family, Plantaginaceae, for centuries. 

Veronica was the basis for the proposed
family name Veronicaceae.  This name was
preferred by many, because at least it was
more symbolic of the types of flowers found
in most members the family. Plantaganiceae,
however had priority.
The name Veronicaceae was originally suggested for the new family (Olmstead et al. 2001), but according to the rules of taxonomy, an existing family names take precedence over a new family names, and since Plantaginaceae has been recognized as a family much longer than the Veronicaceae, the older name must prevail, and so the Snapdragon Family is now officially the Plantaginaceae.  Plantago represents a tiny specialized twig of the snapdragon family tree, and it is too bad to name this family of colorful, distinctively shaped flowers for such an atypical member. This tail-wagging-the-dog result seems counterintuitive and hard to accept, but for now that's the way it is.

The old Scrophulariaceae was polypheletic, and in order to create families that represented genuine evolutionary relationship, it had to be broken up.  Other traditional families have suffered a similar fate, including the Lily Family, Liliaceae.  Daylilies, for example, are no longer in the Lily Family - but that's another story.

Reference: Olmstead, et. al. 2001.  The Disintegration of the Scrophulariaceae.  American Journal of Botany 88:348-361.