Of the large number of plant species which currently inhabit the earth, most (about one-quarter million) are angiosperms, plants that reproduce by means of flowers. The famous naturalist Charles Darwin called angiosperms “an abominable mystery.”
Even today, much remains to be learned about the ancestry of angiosperms and when and where angiosperms first appeared. It is now believed that the first extant angiosperm is a single species of the genus Amborella. This plant, found on an island in the South Pacific, is a shrub with cream-colored flowers.
All angiosperms are assigned to the phylum Anthophyta. Within this phylum, considerable diversity occurs. However, two large lineages have traditionally been recognized: class Monocotyledones (monocots) and class Dicotyledones (dicots).
Themonocots include grasses, cattails, irises, lilies, orchids, and palmtrees. The dicots include the vast majority of seed plants: herbs, vines, shrubs, and most trees (cone-bearing trees are not angiosperms).
The terms “monocot” and “dicot” reflect the number of cotyledons, one or two, respectively, possessed by seeds of the plants. Acotyledon is the central portion of a seed embryo to which the epicotyl (immature shoot) and radicle (immature root) are attached.
However, one need not examine the seed of a particular flowering plant in order to assign it to the correct class. Fortunately, each angiosperm possesses a “syndrome” of features, any one of which may be used for the purpose of classification.
Monocots have leaves with parallel veins: The large, easily visible veins are parallel and extend the length of the (usually) linear leaves. In contrast, leaves of dicots are net-veined: The veins branch repeatedly as they extend into the various portions of the leaf.
Vascular bundles (clusters of conducting cells) within stems of monocots are scattered, in contrast to those of dicots, which are arranged into a cylinder. These are best seen in a cross-section of the root. Furthermore, stems of monocots rarely produce wood, whereas those of many dicots (trees and shrubs) increase their diameter yearly, as wood accumulates within them.
Flowers of monocots typically have their parts arranged in threes and are said to be “3-merous.” An example would be a lily flower, which has these parts (designated fromthe outside toward the center of the flower): three sepals, three petals, six stamens, and one pistil.
Flowers of dicots have parts arranged in fours or fives. Also, pollen grains produced by flowers of monocots each have one pore in the outer covering, as compared to three pores in the case of dicot pollen grains.
Thus, it is apparent that nearly any part of a flowering plant can be used to place it into the correct subgroup of angiosperms. However, it is the flowers and leaves that are most easily considered (not requiring microscopic examination).
Like all areas of science, botany is subject to modification and reinterpretations as a result of the accumulation of new information. One group of plants traditionally considered to be dicots, the magnoliids, have long been problematic. They have some features of dicots, but their floral parts are free (unattached to one another) and arranged spirally.
Also, they produce pollen grains, each of which has only a single pore. These features are characteristics of monocots. These traits have thus caused thosewho study angiosperms to place them into a new, third category, the classmagnoliids.
This most primitive group of angiosperms is thought to be ancestral to both of the other classes. The southern magnolia, Magnolia grandiflora, with its huge, white flowers, is a good example of this class. The name eudicots is applied to the still-large group that includes the remainder of the dicots.