Symmetry 173 



Among some families of angiosperms this same genetic or habitual 

 anisophylly occurs. Thus in Pellionia ( Urticaceae ) , in Centradenia 

 ( Melastomaceae, Fig. 7-12), and in Columnea ( Gesneriaceae ) one mem- 

 ber of each pair is a large typical foliage leaf but the other, directly 

 opposite it, is a small bract-like structure. These differences are ap- 

 parently unrelated to environmental conditions. It is noteworthy, how- 



Fig. 7-11. Dorsiventral (flat) shoot of 

 LycQpodium. At right is a branch grown 

 in the dark, which is radially symmetri- 

 cal. ( From Goebel. ) 



Fig. 7-12. Anisophylly in CeftfrL'denia. 

 Leaves are opposite but one member of 

 each pair is much larger than the other. 

 Only the larger ones have axillary shoots. 

 ( From Goebel. ) 



ever, that this anisophylly is most extreme in horizontal shoots of such 

 plants and is much reduced in those which grow more nearly vertically. 

 On flattened plagiotropic shoot systems there are often changes in the 

 pattern of symmetry that are more complex than anisophylly. In such 

 shoots, for example, many leaves are asymmetric, but in a regular and 

 predictable fashion (Fig. 7-13). Thus in horizontal branches of elm and 

 linden, the inner half of the leaf, directed toward the apex of the shoot, 



