388 VEGETABLE PHYSIOLOGY 



not only a slowing of the responsive movement of the stem, 

 but in some cases an actual reversal of it. Both movements 

 seem to have for their object the placing of the shoot in a posi- 

 tion most favourable to the plant under the then present con- 

 ditions of illumination, for too brilliant a light is deleterious. 

 This reversal of the normal curvature in the presence of an 

 excessive stimulation can be seen with great clearness in the 

 case of certain sporangiophores of Phycomyces, as was first 

 shown by Oltmanns. 



A somewhat similar response to the influence of a lateral 

 light is exhibited by many unicellular organisms. When 

 these are exposed to oblique illumination they take up a 

 definite position with regard to the incident rays, placing 

 their long axis parallel to them if the light is weak, and at 

 right angles to them if it is intense. This behaviour is 

 known as phototaxis ; it is exhibited by the zoospores of 

 many of the Algae and by certain Desmids. 



Before leaving the subject of the effect of a lateral light 

 in inducing these movements, we may point out that the 

 phenomena of heliotropism and apheliotropism must be 

 distinguished from those of photo-epinasty and photo - 

 hyponasty, which were alluded to in the last chapter (p. 371). 

 The difference is easily seen, for in the latter cases the 

 result of the access of the light is the same, whatever be 

 the portion of the organ stimulated. The thallus of Mar- 

 chantia becomes convex on the dorsal and concave on the 

 ventral side, whether the light impinges on the one or the 

 other. In the case of a heliotropic curvature the side 

 which is stimulated always becomes concave ; in that of 

 an apheliotropic one the stimulated side becomes convex. 



GRAVITATION. The force of gravitation exerts an influence 

 upon plants which in some respects resembles that of lateral 

 illumination. Most stems grow vertically upwards into the 

 air ; primary roots grow vertically downwards into the 

 soil. A few organs, among which may be mentioned certain 

 rhizomes and the runners of many plants, grow at right 

 angles to the direction of gravity. When one of these is 



