12 PART I. THE MORPHOLOGY OF PLANTS. [ 2. 



course of its development. For instance, the originally radial 

 lateral shoots of many trees and shrubs eventually become isobi- 

 lateral in consequence of twisting or torsion, by which the leaves 

 come to lie in two rows, one on each flank of the branch (e.g. Elm, 

 Hornbeam). Again, originally radial lateral shoots may become 

 dorsiventral. Thus, in many Coniferae, the lateral shoots become 

 dorsiventral, as is shown by the position and the size of the leaves. 

 The leaves twist so that their ventral surfaces are directed up- 

 wards, and the leaves on the upper are considerably shorter than 

 those borne on the under surface of the branch. The same thing 

 occurs in many flowers (e.g. Epilobium) ; they are at first radial, 

 but they become dorsiventral. Similarly, an isobilateral member 

 may become dorsiventral. The most familar instance of this is 

 the development of the dorsiventral shoot of Marchantia from the 

 isobilateral gemma ; again, many dorsiventral leaves become 



Pis. 4 Isobilateral gemma of Marchantia in tranverse section ; a, cells capable of 

 developing into root-hairs ; e, growing point of one side ; d, the margin of tlje gemma 

 projecting on the further side of the depression in which the growing-point is situate I 

 (after Pfeffer). 



isobilateral under certain conditions of exposure to light. Further, 

 creeping shoots (e.g. Acorus and Butomus) may, in consequence 

 of torsion, bear all their leaves on the upper (dorsal) surface, 

 whilst the ventral surface bears roots. Again, an isobilateral 

 member may become radial ; a stem which bears leaves originally 

 in two rows, and is therefore isobilateral, may subsequently bear 

 leaves in many rows, thereby manifesting its radial symmetry. 

 Finally, a dorsiventral member may become radial. For instance, 

 the fertile branches of the dorsiventral Marchantia- thallus are 

 radial ; again, the branches of Phyllocladus trichomanoides, are 

 under ordinary circumstances dorsiventral, but if they are espe- 

 cially vigorous they may become radial. 



The causes which determine the symmetry of the body or of a 



