138 RELATIONSHIPS OF SYMMETRY 



2. The oblique direction of the lateral axes is influenced by the 



chief axis. They all curve away from the chief axis. 



3. Internodal torsion of the axis of inflorescence as in other plagio- 



tropous shoots. 



4. Positive geotropism of the flower-stalk. 



We see in the dorsiventral inflorescences that nature seizes, so to 

 speak, the good where it finds it ; how this originates is a matter of no 

 importance. In definite circumstances it is of advantage that the flowers 

 be directed towards one side, and this may be brought about partly by 

 curvatures heliotropic, geotropic, plagiotropic position of the axis of 

 inflorescence partly by laying down of the flowers upon one side ; and 

 that unilateral illumination may bring about an internodal torsion which 

 results in decussating leaves taking a two-rowed position, I have already 

 shown in Urtica dioica l . 



In what I have said it has been assumed as probable that the dorsi- 

 ventral inflorescences have proceeded from radial ones. We see too 

 that in the inflorescences which are unilateral this process does take 

 place in the course of the development. We do not know the cause or 

 causes (because the ways may have been different) which have con- 

 ditioned the origin of the inflorescences which are from the first laid 

 down as dorsiventral. In the Leguminosae the absence of the flowers 

 upon one side of the inflorescence has been explained 2 by the assumption 

 that their development has been hindered by the pressure of the chief 

 axis against which the inflorescence lies closely adpressed on one side 

 (see Fig. 83), and in Trifolium rubens and Medicago sativa, which have 

 only a portion of the base of the inflorescence wanting flowers, a similar 

 explanation has been given. But it is quite possible that the causal 

 relation may have been the converse that no flowers having arisen 

 their room was used in a better way and that thus the adpression occurred. 

 It can be nowhere proved that such coarse mechanical relationships as 

 those of pressure exercise so far-reaching an influence upon the configura- 

 tion. Amongst the Leguminosae, too, there are species of Trifolium 

 in which the laying down of the flowers is at first unilateral and then 

 gradually the whole inflorescence is laid claim to without any relationships 

 of pressure whatever having come into operation. The peculiarity of 

 the inflorescence is much more probably here fixed from the beginning in 

 the form of its vegetative point ; the absence of the flowers upon the 

 one side may rather stand in connexion with the fact that their function 

 having become enfeebled, abortion was their fate. Still, as I have said, 

 the way in which this has been brought about has yet to be discovered. 



1 Goebel, in Botan. Zeitung, 1880, p. 843. 



2 First of all by Godron, Observations sur les bourgeons et sur 1'inflorescence des Papilionace'es. 

 Nancy, 1865 



