|This Ficus tree has a broad canopy of leaves and|
a broadly spreading root system connected by
a narrow trunk. Photo by William A. Essig.
|Coral polyps have a wheel-like, or radial symmetry, |
when viewed from the top. You could slice it into
identical halves by cutting through the center at any
|A higher animal, like a lizard, can be split down|
the center, from head to tail, to create two mirror-
image halves. Cutting it any other way creates
very different looking halves.
What about plants - do they have symmetry? Plants don't move, so symmetry would have a different meaning, if it exists at all. In fact it does, but it's more complicated. Plants exhibit both radial symmetry and bilateral symmetry, often at the same time. In flowers, it has a lot to do with pollination strategy. A sunflower head has radial symmetry, allowing small insects to land on top, while a snapdragon flower has bilateral symmetry, inviting large bees to enter from the side.
The whole plant can have one or both symmetries also, as discussed by naturalist Francis Halle (2002). Plants that are anchored to a single spot, like trees, exhibit an overall symmetry that is roughly radial. The weight is more-or-less evenly balanced around its central axis, which extends down the trunk into a woody taproot system. A tree could hypothetically be spun around its central axis without affecting its environmental orientation. Like a coral polyp, a tree gathers resources -sunlight and carbon dioxide - that are widely dispersed, and so must spread a wide net from its central trunk. Of course the symmetry is not exact because of the random nature of tree branching, but overall the crown of a typical tree is a rounded dome. This symmetry is more obvious and exact in something like a tree fern or a single-stemmed palm tree, like a date palm, though these do not have woody taproot systems.
|A woody tree or shrub has |
an hour-glass symmetry, as
the massive crown is
balanced by an
equivalent woody root
|Palms are not woody, but their|
crown of massive leaves is balanced
at the other end of the trunk by a mass of a
Creeping ferns, gingers, or irises, on the other hand, have a roughly bilateral symmetry. Each stem, or rhizome, "travels" horizontally through the soil over time, with the apical meristem creating new tissues as the older end of the rhizome ages and disintegrates. As a rhizome grows forward, it sprouts roots on its lower side and a series of leaves or leafy shoots on its upper side. Such a plant does in fact move, albeit slowly, and through branching can come to cover a large area and live indefinitely. One can make a longitudinal cut along the center of a rhizome, and the two halves will be more-or-less the same. Spinning such a plant around its stem axis would result in leaves and roots alternating in the wrong environment.
|A liverwort body is a simple, bilaterally symmetrical ribbon|
that grows at its tip. The specimen is Pallavacinia lyellii
growing in central Florida.
|Sphagnum moss forms miniature forests of upright, |
tree-like shoots that emerge from underground stolons
Ancient vascular plants, like ferns and club mosses, grow from horizontal rhizomes, but sometimes have radially symmetrical upright shoots. Creeping plants all produce new (adventitious) roots from their stem tissues as they go. The evolution of real trees from such ancestors was a remarkable event. Invention of secondary growth and wood was only part of the story. The more radical change was the reorientation of the plant into a single, vertical, bipolar axis that included a woody branching root system extending down directly below the trunk, as pictured above.
|The trunk of a banana plant is a pseudostem of soft|
leaf sheaths rising from an underground rhizome. The
leafy shoot is roughly radially symmetrical. From Brown,
The Plant Kingdom, 1935, Fig. 91.
Monocots reinvented creeping bilateral symmetry in their primary stems, but often have radially symmetrical upright shoots. New banana shoots, for example, sprout from short rhizomes that have budded off of an older plant, and these upright shoots have leaves spreading out in a circular pattern that minimizes them shading each other. Many palms, those referred to as "clumping," likewise spread by rhizomes, but send radially symmetrical shoots upwards.
|In this historic photo from Singapore,|
a traveler's palm, Ravenala madagascariensis,
has created a massive fan-shaped crown,
giving it a bilateral symmetry.
Reference: Halle, F. 2002. In praise of plants. Timber Press. Portland, Oregon.