22.1Basic System of Plants
Plants have only three basic organs: the root, which works to absorb water and mineral nutrients; the leaf, which is the primary site of photosynthesis in plants; and the stem, which supports leaves and connects them to the root. Figure 22-1 shows how these organs make up individual plants in the Dicotyledoneae in angiosperms. This section describes the basic principle of the common organ structure of all seed plants and the essential characteristics of the plant system.
In aerial parts, the stem together with the leaves attached to it can be taken as one unit of the plant body. This structure is called the shoot, the tip of the shoot is called the apical bud, and there are lateral buds at the base of the leaves. When the lateral buds grow, they become new shoots called offshoots. Then again, shoots grow from the lateral buds of the offshoots, and branching is repeated in this way.
In underground parts, lateral roots develop from within the taproot, which originated from the radicle of the embryo. Lateral roots also produce their own lateral roots, and this is repeated to form the complicatedly branched root system. The taproot and stem, which is the main axis shoot, are connected via the hypocotyl. The fact that the hypocotyl is formed during embryogenesis differentiates it from the stem, which develops from the bud. However, functionally, the hypocotyl is similar to the stem.
There are also some roots that develop from organs other than roots. Such roots are called adventitious roots. Figure 22-1A shows an adventitious root growing from the stem node. Adventitious roots, taproots, and lateral roots are quite similar, differing only in that they originate from different organs. Adventitious roots can also branch by developing lateral roots to form a complex root system. The fibrous root system of monocotyledons is one example of a root system based on the adventitious root.
An interesting aspect of plant morphology, which can be seen in both the aerial and the underground parts is the nested branching pattern formed by the repetition of the same unit (the shoot in aerial parts, and the root in underground parts). This characteristic is unique to plants, which consistently form and add organs throughout their life, and this branching pattern is thought to be the key to our visual perception in recognizing a plant. Even a simple pattern based on a very simple branching rule will eventually start looking like a plant after repeated branching (Figure 22-1B).
Figure 22-1C shows the basic system of plants from the viewpoints of polar characteristics and symmetric properties. Both the stem and root are radially symmetric axial organs with polarity that runs from the base to the tip along the axis. The polarity in the radial direction runs between the adaxial (toward the center of the axis) and abaxial (away from the center of the axis) regions of the plant. Leaves are generally zygomorphic, but when we look at the shoot as a unit consisting of leaves and a stem, it is radially symmetric. Moreover, the whole plant as an organism demonstrates radial symmetry around the vertical axis that runs from the tip of the main axis shoot to the tip of the taproot. While most animals are bilaterally symmetric, radial symmetry is another important characteristic of the appearance of plants.