5.2: Organs and Organ Systems
Vegetabilia (Figure 6.1.1) have three different types of body construction (Figure \(\PageIndex{1}\)). The most primitive plants have thallus body, more advanced is the shoot (unipolar) plant body, and most land plants have the bipolar plant body. The thallus flat , non-differentiated body plant body is flat, similar to leaf but do not differentiated into particular organs. Most gametophytes (except true mosses) have this type, and also few sporophytes (which mostly are reduced water plants). Shoot (unipolar) plant body consists only of branching shoots, roots are absent. This is typical to all Bryophyta sporophytes, mosses (Bryopsida) gametophytes, and also to sporophytes of Psilotopsida (whisk ferns). Finally, bipolar plant body present has both shoots and roots (Figure 5.3.1). Most bipolar plants have shoots consist of stems and leaves, but this is not an absolute requirement since young plant stems are normally green and can do photosynthesis.
Typical organs of bipolar plant are stems (axial aerial organs with continuous growth), leaves (flat lateral organ with restricted growth), roots (axial soil organ modified for absorption) and floral units (FU) which are elements of the generative system (fructifications) such as a pine cone or any flower.
Buds, fruits, seeds and specific to seedlings hypocotyl and epicotyl are non-organs for different reasons: buds are just young shoos, fruit is the ripe flower, hypocotyl is a part of stem between first leaves of the seedling (cotyledons) and root (i.e., stem/root transition place), epicotyl first internode of the stem is first internode of stem (Figure \(\PageIndex{2}\)), and finally, seed is a chimeric structure with three genotypes so it is impossible to call it “organ”.
Root, stem, leaf and FU are four basic plant organs (Figure \(\PageIndex{3}\)) which in bipolar plant could be grouped in root and shoot system; the latter is frequently split into generative shoot system (bearing FU), and vegetative shoot system (without FU).
Vegetative shoot system usually consists of main and secondary shoots; shoots contain terminal buds, axillary (lateral) buds, stem (nodes and internodes) and leaves. We will start from leaves.