Plant Ecology of the Sonoran Desert Region - Design with the Desert: Conservation and Sustainable Development - page 150

Robertson calls diaphanous plants. Their stems and foliage are so fine-textured and sparse
that the eye has difficulty focusing on them instead of looking right through them.
These tactile, olfactory, and visual experiences offer clues to desert plants' adaptations
to their rigorous environment. Before exploring these adaptations, it is necessary to
understand something about plant structures, functions, and classification.
8.2 Basic Plant Anatomy and Classification
Many people mistakenly identify ocotillo, agaves, African euphorbias, and numerous
other plants as types of cacti because of their succulent or spiny stems. In fact these
plants, despite their almost identical in outward appearance, are unrelated to each
other and to cacti. Similarities of outward appearance are often examples of convergent
evolution and are not reliable indicators of relationship. Convergent evolution results
when unrelated organisms develop similar adaptations to similar environmental
conditions. The sexual parts of plants (flowers and fruits) are used almost exclusively to
determine their interrelationships. The parts of flowers and fruits are easier to identify
and describe than the vegetative organs (leaves, stems, and roots), and these complex
floral patterns remain more readily traceable as they evolve. On the other hand,
leaves and other vegetative parts can also be measured, but it's difficult to determine
relationships among plants from such measurements. Moreover, qualitative vegetative
characters are difficult to describe precisely even when the overall appearance (gestalt)
is distinctive. For example, nearly every hiker knows poison-ivy on sight. But try to
describe the foliage so precisely that someone who has never seen one can distinguish
it from skunk bush ( Rhus aromatica ). It's quite difficult to describe the leaves' different
shades of green, degrees of hairiness, and the scalloping of the margins, especially
if you lack the minutely detailed vocabulary of the botanist. For example, pubescent,
puberulent, lanate, villous, hirsute, hirsutulous, ciliate, tomentose, strigose, pilose, and
hispid are just some of the terms describing different degrees of hairiness. Vegetative
parts are also more plastic, that is, they vary greatly under environmental influences.
The leaves of brittlebush grow much larger and greener in shade or during rainy periods
than in sun or drier conditions.
The complexity of flowers and fruits creates distinctive patterns that can be character-
ized exactly. Petals, stamens, and other parts can be counted, their lengths and widths
measured (and these are usually less variable than the dimensions of a leaf). Where the
stamens are attached to the petals (or other part) can be described unambiguously. For
example, a flower that has many (more than 10) petals and sepals that intergrade into one
another, many stamens (usually hundreds), a multilobed stigma, and an inferior ovary
unequivocally identifies a member of the cactus family. All 2000 species of cacti possess
some variation of this basic pattern, and no other plant group does.
To recognize floral patterns you must be able to identify the parts of a flower
Figure 8.1. The following drawing identifies the anatomy of a generalized flower.
8.2.1 Classification and Plant Identification
In the game “Twenty Questions” players attempt to identify an unknown by asking the
person who knows the answer a series of yes-or-no questions. If done well, 20 questions