498 LECTURE XIX. 



inasmuch as under the given conditions the incident rays of 

 light may be regarded as falling vertically downwards, the 

 directive influence of light upon these organs is likewise such 

 as to cause them to maintain a vertically upward direction of 

 growth. 



But it occasionally happens that these organs grow under 

 conditions other than these, as, for instance, when they grow in 

 hedges. One side of the organ is then more exposed to light 

 than the other, in other words, the light falls not vertically but 

 obliquely upon them. Under these circumstances gravity and 

 light act somewhat antagonistically; the organs tend to grow 

 vertically in virtue of their negative geotropism, and to grow 

 obliquely in virtue of their positive heliotropism ; their direc- 

 tion of growth will depend upon the relative strength of the 

 geotropic and heliotropic effects produced. In the majority of 

 cases, as mentioned in a previous lecture (p. 430), the heliotropic 

 effect is the greater, so the organs grow obliquely towards the 

 light; but in some cases (p. 431) the geotropic effect is the 

 greater, so they grow erect. 



The relative strength of the geotropic and heliotropic 

 effects produced in these organs depends upon their relative 

 geotropic and heliotropic irritability, and upon the intensity of 

 the incident light. This has been investigated in some cases. 

 Muller-Thurgau observed, for instance, that when a shoot of 

 Helianthus was placed horizontally and light was directed 

 upon it from below, it curved upwards, whereas a shoot of 

 Fritillaria remained straight under similar conditions. The 

 intensity of the light being the same in the two cases, it is 

 clear that either the geotropic irritability of Helianthus is 

 greater than that of Fritillaria, or that the heliotropic irritability 

 of Fritillaria is greater than that of Helianthus. It appears 

 that, as a rule, the heliotropic irritability of the primary shoots 

 of seedlings is greater than their geotropic irritability, for, 

 when they are placed horizontally and are illuminated from 

 below, they curve downwards, as von Mohl and Muller- 

 Thurgau have found. Wiesner has carefully investigated this 

 subject with special reference to the intensity of the light. 

 When the primary shoot of a Bean-seedling growing vertically 



