Chap. Ill] LIGHT 57 



when there is a sufficient supply of organic nourishment, and flower-buds 

 that are already formed usually die soon ; any flowers that may be formed 

 are usually of abnormal shape and either coloured slightly or not at all. 



Etiolated plants are seldom found wild, although they sometimes occur 

 in caves. For instance, in the well-known Guacharo cave near Caribe 

 in Venezuela we found the ground covered with patches of dense etiolated 

 vegetation up to half a meter in height, which had sprung up from the 

 dung of the Guacharo birds, the only inhabitants of the cave. 



4. INTENSITY AND QUALITY OF LIGHT. 



The action of light on plants is either invigorating or restricting, creative 

 or destructive, according to its intensity and according to the precise 

 physiological function involved. The intensity of illumination, at which 

 one or other action commences, varies, as in the case of heat, in different 

 species of plants; there are however no exact data on this subject. 



Growth in length of stems and roots is at its optimum when light is 

 totally excluded. Even light of very weak intensity exercises a retarding 

 influence in this respect, while light of high intensity brings the process 

 to an absolute standstill. 



The growth in area of leaves in darkness is very slight, but attains its 

 optimum in light of very moderate intensity. Any further increase in 

 the illumination retards and eventually arrests the process. The optimum 

 intensity of light for growth in thickness of leaves is considerably higher 

 than for the growth of the leaf-surface, so that strongly illuminated leaves 

 are small and thick. 



The development of vegetative buds of trees does not commence until 

 the light has reached a definite intensity which is not very low ; weak 

 illumination causes the death of branches, the so-called cleaning of the 

 stems of the trees 1 . 



The action of light on the origin and development of the reproductive 

 organs, which has been closely studied by Sachs, Moebius, and Vochting 

 in the higher orders of plants, and by Klebs in the lower orders, has 

 an important bearing on phyto-geography. Vochting in particular was 

 able to prove that under a weak illumination the formation of flowers, in 

 numerous phanerogams, was either entirely prevented or only incompletely 

 achieved. In a room with one window looking ENE. flower-buds were pro- 

 duced either very scantily or not at all, whilst vegetative growth remained 

 normal or was even abnormally luxuriant, in the case of Mimulus 

 Tilingi. Buds that were already formed died at early stages ; others pro- 

 duced degenerate and malformed flowers ; chasmogamous flowers became 

 cleistogamous, for the arrest in development usually affected the perianth 



1 Wiesner, V. 



