150 The Phenomena of Morphogenesis 



and most protozoa, show axial symmetry, often complex in character; and 

 all typical examples of symmetry in higher plants are those manifest 

 around an axis or a longitudinal plane. Three general types of such sym- 

 metries are recognized: radial, bilateral, and dorsiventral. All the com- 

 mon patterns for the structure of plants and their organs are based on 

 these symmetries. 



RADIAL SYMMETRY 



In this type there is an axis of rotation around which symmetry is uni- 

 form. There may be one or two evenly spaced longitudinal planes of 

 symmetry, as in stems with distichous and with opposite-leaf arrange- 

 ments, or these may be almost infinite in number in stems with spiral 

 symmetry. Radial symmetry is present in vertically elongated axes such 

 as those of the main stem and primary root and in many flowers and fruits. 

 It is therefore much commoner in plants than in animals, since most of 

 the latter show dorsiventrality, and it is regarded by many as the most 

 primitive type of symmetry, at least in vascular plants, since their first 

 axes were presumably vertical. 



In Lower Plants. Individual cells often show radial symmetry regard- 

 less of their orientation, as in Spirogyra and Chara. Many plant bodies in 

 the thallophytes have this type of symmetry, a familiar example of which 

 is the "mushroom" form of sporophore in the fleshy fungi. Many red algae 

 have radial thalli, as do some brown algae. Most true mosses also are 

 radial. 



In Roots. Almost all roots are radially symmetrical. This symmetry is 

 shown in the straight and evenly spaced rows of lateral roots and in the 

 characteristically radial primary vascular structures, in which arms of 

 xylem typically alternate with bundles of phloem in a star-shaped pattern, 

 with lateral roots arising opposite the xylem arms. 



In two respects the expression of symmetry in the root is different from 

 that in the stem. Roots, even horizontally growing ones, are usually 

 strictly radial and (save for a few cases such as air roots of orchids) 

 show no dorsiventrality, regardless of their orientation, whereas horizon- 

 tal stems commonly do show this. Roots also have very little twisting or 

 spirality in their internal or external structures, such as most stems dis- 

 play. These two differences emphasize again the fundamental diversity in 

 developmental behavior of root and stem which is evident in many other 

 respects. Whether these differences are inherent or are due to the radical 

 differences in the environment in which roots and stems usually develop 

 is an interesting morphogenetic problem. 



In Shoots. The symmetry of shoots, and particularly that shown by the 

 arrangement of leaves (phyllotaxy), has attracted more attention than 



