22 



BOTANY 



planes passing through the centre (or axis). The simplest type 

 is seen in a globular organism, like Volvox. A cylindrical stem, 

 like the trunk of a Pine, is also radially symmetrical; and the 

 so-called "regular" flowers of a Buttercup or Rose show a radial 

 arrangement of the floral organs. Radial symmetry is also appar- 

 ent in the arrangement of the leaves on the shoots of many plants ; 

 e.g. most Mosses, shoots of Oak, etc. 



Parts are bilaterally symmetrical when they can be divided into 

 similar halves in one or two planes only. A few unicellular plants, 

 like most Desmids (Fig. 3, C) and Diatoms (Fig. 13, A), are bilater- 

 ally symmetrical; and among the higher plants shoots with two- 

 ranked leaves, leaves themselves, and the so-called "irregular" 

 or " zygomorphic " flowers, e.g. Orchids, Snapdragon, etc. are 



FIG. 13. Bilateral symmetry. A, two views of a Diatom, Pinnularia viridis. 

 B, zygomorphic flower of Viola cucullata. C, leaf of Ivy. 



familiar examples. Bilateral structures may be either Iso-bilateral 

 or Dorsiventral. In the former case, e.g. Desmids, vertical leaves 

 of Iris or Manzanita, phyllodia of Acacia, etc., the organ may be 

 divided into equal parts by either a horizontal or a vertical plane. 

 Dorsiventral structures can be equally divided by a vertical plane 

 only, e.g. ordinary horizontal leaves ; the thallus of most Liverworts, 

 etc. 



ORGANS OF VASCULAR PLANTS 



With few exceptions the body (Sporophyte) of a vascular plant 

 always shows a clear separation into root and shoot; and the latter 

 normally consists of the stem and leaves. There are also, very often ; 

 developed from the surface various kinds of Trichomes, hairs and 

 scales; finally, the sporangia (pollen-sacs, ovules) or reproductive 

 structures, are developed, usually as appendages of modified leaves. 



