590 LECTURE XXI. 



us is an illustration of this general principle. But in some 

 cases, at least, the excessive elongation in darkness of the 

 shoot-organs of plants destitute of chlorophyll has an imme- 

 diate significance. It was mentioned in a previous lecture 

 (p. 393) that the spores of certain Fungi are not developed 

 when the plant is kept continuously in darkness. The excessive 

 elongation of the stalks of the fructifications of these Fungi in 

 continuous darkness is the expression of the same tendency 

 as the excessive elongation of the stems of normally green 

 plants : it is the expression of the search after light, though 

 the end to be attained is different in the two cases. In the 

 one case light is essential to the development of the spores ; 

 in the other, to the performance of the processes of construc- 

 tive metabolism. 



We may now go on to discuss the significance of the 

 various forms of changes in position. Beginning with the 

 phenomena induced by light we will take as our first illus- 

 tration the case of a positively heliotropic shoot. The effect 

 of an obliquity in the incidence of the rays of light upon 

 such an organ is to cause it to place its long axis in the 

 direction of incidence of the rays, the apex being directed 

 towards the source of light. Such a curvature of the stem 

 necessarily alters the position of the leaves, the alteration 

 being such that the upper surfaces of the leaves are placed 

 perpendicularly to the direction of the incident rays. In 

 this position the leaves are fully exposed to the incident 

 light, and are under the most favourable conditions for 

 the performance of their important metabolic processes. 



But the position of leaves with regard to the direction of 

 incidence of light is not altogether determined by the di- 

 rection of growth of the stem which bears them ; for we 

 have seen (p. 445) that the fixed light-position of leaves is 

 different in different plants. In endeavouring to explain 

 the differences in the fixed light-positions of leaves, we must 

 bear in mind that light not only promotes the constructive 

 metabolic processes, but that, if intense, it also causes de- 

 composition of chlorophyll (p. 265). It is important, there- 

 fore, that the position of a leaf should be such that whilst 



