go RELATIONSHIPS OF SYMMETRY 



in sharply defined categories, and the following arrangement is merely 

 a convenient one : 



A. Creeping and climbing shoots. 



B. Dorsiventral lateral shoots. 



C. Anisophylly. 



Anisophylly is found both in creeping and climbing chief shoots and 

 in dorsiventral lateral shoots, and I only treat of it here as a special 

 category because of its common occurrence. Asymmetric leaves are 

 another peculiarity of many dorsiventral shoots, and they will be spoken 

 of in the chapter on the relationships of symmetry of leaves (see p. 114). 



A. CREEPING AND CLIMBING SHOOTS. 



The dorsiventrality of these shoots shows itself in the presence of 

 the roots upon the ventral side, turned away from the light, in the manner 

 in which we find them in the thallus of a liverwort, on the prothallus 

 of a fern, and also in many higher plants. The relationship of this 

 construction to light will be discussed in the Fifth Section. A second 

 indication of the dorsiventrality of these shoots is the 'tendency' to 

 displacement of the leaves towards the upper side, whilst the lateral 

 shoots remain upon the flanks, and there is a remarkable agreement in 

 the occurrence of this in the most different cycles of affinity of plants. 

 We can, as so often happens, distinguish two cases here either this 

 position of the organs is acquired in course of their individual development, 

 or it is fixed from the beginning at the vegetative point. 



Two examples of bilateral shoots with distichous phyllotaxy may 

 be cited in illustration of the first case. Monstera deliciosa, one of 

 the Aroideae, possesses climbing shoots the leaves of which are so 

 displaced to the dorsal side, chiefly through torsion of the internodes, 

 that they often appear as if they were in one row. We find the same 

 in the creeping shoots of Acorus and Butomus. In Butomus umbellatus 

 the creeping shoot has an erect terminal bud in which the leaves are 

 in two rows and the primordia of the lateral shoots stand in the median 

 plane of the leaves ; but on the prostrate portion of the rhizome the leaves 

 stand in two rows closely approximated upon the dorsal side, and the 

 lateral buds are found upon the flanks at the lower edges of the leaves, 

 while the ventral side appears to be quite free from leaf-insertions and 

 bears only roots. 



In the second case the position of the organs is quite similar. 

 Caulerpa prolifera, one of the Siphonieae, possesses a creeping stem 

 which bears upon its ventral side ' roots,' on its flanks twigs, and on its 

 dorsal side 'leaves' (Fig. 47). The floridean alga Herposiphonia shows 

 the same thing, having two rows of ' leaves ' upon its dorsal side, lateral 



