THE ANGIOSPERMAE : STEMS S37 



different poles, at the stem apex and the root apex respectively, which are 

 permanent and are replaced if destroyed. 



Lateral symmetry relationships are classified under three headings : — . 



1. Radial Symmetry, when a part is organised equally on all radii 



round the longitudinal axis. 



2. Bilateral Symmetry, when a part has two distinct sides, i.e., is not 



organised equally on all radii round its axis. If both sides are 

 alike it is called Isobilateral, or if they are unlike it is called 

 Anisobilateral. 



3. Dorsirentral Symmetry, when a bilateral part has two differently 



organized faces, i.e., possesses a distinct top and bottom or 

 j back and front. 



Radial organs are also, for the most part, orthotropic, that is, vertically 

 placed, while bilateral and dorsiventral organs are mostly plagiotropic , 

 that is, horizontal or inclined in position. 



This latter distinction points to the fact that symmetry is not entirely a 

 matter of inner organization, but, as has been shown by many experiments, 

 can be fairly easily modified by external influences, especially bv light and 

 gravity, which are the factors most affected by differences in the position 

 of an organ. For example, many bilateral shoots, such as those of the 

 flattened Cactus, Opimtia, develop with radial symmetry in the dark and 

 the bilateral side shoots of Tradescantia become radially symmetrical, like the 

 main axes, if grown free from gravitational effects on a klinostat. The dorsi- 

 ventral internal structure of many leaves can be changed into an equifacial 

 structure by development under conditions of equal illumination of both 

 surfaces, and the anisobilateral growth of some leaves, like those of the 

 Elm in which one half is bigger than the other, can be reduced to equality 

 by severing the vascular supply to the larger side. 



Such experiments show that symmetry is frequently the imprint of growth 

 conditions, but these conditions operate upon nutritional differences, growth 

 tendencies and " correlations," that is, influences due to the effect of one 

 growing part upon others, which arise from internal causes quite independently 

 of external influences and which vary greatly in the degree to which they are 

 susceptible of external control. The more fundamental such inner tendencies 

 are, which probably means the older they are, the more difficult they are to 

 change. Thus it is practically impossible to change the bipolarity of the 

 axis or to change a bilateral leaf into a radial structure by experimental means, 

 though even such fundamental changes have sometimes occurred during 

 the vast and prolonged course of evolution. 



Branching of the Stem 



Only a minority of Angiosperms have a single axis. The normal habit 

 is for shoots, like roots, to multiply themselves by branching, which makes 

 possible the division of functions between different branches and, above all, 



