Monday, March 26, 2012

The "root" of the root problem

Most plants have roots.  We take that for granted. These important organs grow into the soil (or sometimes tree bark) to provide anchorage and absorb water and minerals.  Some, like carrots and sweet potatoes, swell with food reserves and are edible.  Their structure and function is pretty much the same across the plant kingdom.  So what's the "problem?"

One of my pet peeves as an educator is the clutter of vague and confusing terminology often used to describe the different parts of plants, and the failure to relate terminology to the bigger pictures of plant development and evolution.  One particularly muddy area concerns the kinds of root systems in plants.  Whoa, you're thinking, Botany Professor is way out in Botanygeekland this time.  Bear with me, because there are just two basic kinds of root systems in plants, and they tell us a lot about the fundamental strategies of plants for survival and perpetuation.

Adventitious roots are most easily seen in
an epiphytic orchid.  Though these stems
are more upright, they are modifications of
creeping rhizomes. Roots, stems, and leaves of
orchids are all ephemeral, and periodically
replaced by new organs.
In most ferns, club mosses, and other ancient lineages of spore-bearing plants, as well as in modern monocots, waterlilies, and some eudicots, the main stems, or rhizomes, are horizontal and creep through the soil, putting out new roots as they go.  Their roots have two properties: they are adventitious and they are ephemeral.  Adventitious roots arise individually from the stem of the plant and occasionally from leaves), rather than through branching off of earlier roots.  Ephemeral means that they are temporary, i.e. disposable.  In fact, no part of a fern plant is permanent or woody.  Older parts of the rhizome, as well as older leaves and roots disintegrate as newer organs are generated. 

New adventitious roots emerge from
a young, growing section of rhizome.
The apical bud is to the right.

Such a creeping body plan has been called bilaterally symmetrical by Francis Halle.  Most animals have this kind of elongate symmetry in which the body can be split into two, mirror-image halves, with paired locomotory and sensory organs on either side of the animal.  Such organisms have a front (anterior) end, a rear (posterior) end, a right and left side, a backside (dorsal) and a belly side (ventral).  Think of a centipede creeping along with its many legs on either side. In animals, this is a set of adaptations for efficient forward movement and prey capture, with eyes, brain, and mouth at the front end.

A creeping rhizome can be described in the same terms.  The front end is the apical meristem, the rear end is the decaying part of the rhizome.  Leaves emerge from the top or dorsal side of the rhizome, while roots arise mostly from the belly of the rhizome.  Such a plant is actually mobile, moving slowly through the soil with each year's growth.  They also can branch, creating large colonies of outward-expanding rhizomes segments. 
A rhizome, like this of a Solomon's Seal (Polygonatum sp.), creeps along the ground.  The newest growth, including the primary apical meristem, is to the left.  The round scars, and the structure labeled "1940,"  represent the locations of ephemeral leaf and flower-bearing shoots.  The older growth from 1937 to 1939 is to the right.  Adventitious roots have emerged over time directly from the stem tissues of the rhizome.  From Transeau et al.,  Textbook of Botany, 1940, Fig. 91. 

Why this matters is that early seed plants, which were mostly trees and shrubs, evolved a very different symmetry: a radial symmetry.  This is more like the symmetry of a sea anemone.  From the top, the organs of the animal radiate out from a central point.  A tree, shrub, or cycad has a similar organization.  This kind of organization is more suitable to organisms that stay put, i.e. are sessile.  A tree does not creep along the ground like a fern or ginger rhizome, because, by definition, its growth has been redirected skyward.  It also needs a more massive root system to support that massive upright growth.  The root system of a tree develops through the branching of the original primary root directly beneath the trunk.  It becomes woody over time like the trunk and branches above it, so it is neither ephemeral nor adventitious.  
A woody tree or shrub has a roughly radial
symmetry with an overall hourglass shape. It
is fixed to one spot for life.
A plant embryo consists of an axis with two poles. The primary root develops from an apical meristem at the "south" pole, while the primary shoot develops from an apical meristem the "north" pole. Through branching, the entire trunk and branch system develops from that original embryonic shoot, and the entire root system develops through branching of the primary root.  There remains a relatively narrow zone where the shoot system and the root system meet, giving a tree or shrub an overall hourglass shape.

Though plants are endlessly varied in structure and in their adaptation to particular environments, most fall into one of these two basic symmetries.  There is not, however, a simple pair of terms derscribing the two types of root systems.  They are most often referred to in textbooks as a "fibrous root systems" and  "taproot systems."  This dichotomy is not only vague, but can be misleading as well.

A taproot by definition is a single dominant root, as exemplified most beautifully by a carrot.  While many woody plants begin with a taproot, which develops directly from the primary root of the seedling, most actually branch to the point where the original taproot can no longer be identified.  The term "axial root system" has been used in the past, and is much preferable for those that develop entirely through branching of the lower axis of the embryo. 

A fibrous root system is one consisting of many roots of similar length and thickness, and  forming a thick, broom-like mat.  The adventitious root system of a grass plant or onion bulb fits this model, but the adventitious roots of a climbing Philodendron are not so broom-like.  In some definitions, fibrous is equated with adventitious, but in others, a similar-looking cluster of roots resulting from multiple equal branching of the primary root would also be called fibrous.  In addition, the word "fibrous" is used in a very different context for the presence of strengthening fibers within leaves and stems.  Fibers are not generally present in ephemeral adventitious roots. So this is a poor choice for the typical adventitious root system of monocots and many herbaceous dicots.

Why not just call them "adventitious root systems?"  That, though better, is also somewhat misleading because the word adventitious refers to how a root forms (from a stem or sometimes even a leaf) rather than to its mature form.  Woody plants may produce adventitious roots, but these will typically become woody themselves.  A cutting  taken from a tree or shrub may produce adventitious roots in response to hormone treatment, but as the cutting becomes a new plant, one of the adventitious roots typically becomes a new woody taproot.  Members of the genus Ficus known as banyan trees produce adventitious roots from their main branches, which dangle to the ground and become new woody trunks. These are adventitious root systems, but very different from the ephemeral systems of monocots.

There does not appear to be a fully satisfying term for the "disposable adventitious root systems of  plants with non-woody creeping stems," but it is important to recognize them as a major and widespread  alternative to the permanent woody root systems of trees, shrubs, and carrots (carrots are another story; their thick taproots are woody except that their "wood" has been modified into food storage tissue).  Perhaps something like "fibrous/adventitious" would be the best compromise to accurately identify the distinctive root systems of  ferns and monocots.  If we are to continue to use the word "fibrous" alone for these systems, it must be carefully and consistently defined as adventitious in nature, and contrasted with "axial root systems," whether these are in the form of a taproot or something that superficially resembles a cluster of fibrous roots.

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