218 Causes and Course of Organic Evolution 



vertical direction of growth will now be separated into distinct 

 factors. So the shoot T\'ill place itself as in Fig. 7, b, since the 

 vertically upward apogeotropic or apogravic action will be 

 modified by the unilateral lumic one. 



Fig. 7. — Terminal shoots of Cuphea platycentra. The left-hand figure (a 

 in text) grew under vertical illumination, the right hand figure (b in text) grew 

 under unilateral illumination. 



All sensitive plants — such as many of the Oxalidacese and 

 Leguminosse — are exceptionally valuable for our present study. 

 As illustrations of stimulation and resultant response alike 

 for stems and leaves we may select the sensitive plant (Mimosa 

 pudica), since compounding or separation of response are 

 quickly performed in it or in its ally M. Spegazzini. Photo- 

 illustration 8 a, is taken from a pot-plant that grew in a stove 

 house ^^dth bright diffuse illumination. The stem therefore 

 shows compounded or resultant apogeotropic and heliotropic 

 position response. The leaves are placed transversely or are 

 diageotropic, and all, whether on the stem side toward the 

 incident light or on that away from it, are so tilted at varying 

 angles that the incident rays fall at right angles to the surface. 

 So we may say that these are diaheliotropic. But, if a plant 

 be inverted, the leaves inside an hour or two have undergone 

 a striking angular rearrangement. But, for our present pur- 

 pose Fig. 8, b that was made two days after inversion, but other- 



