72 



JOUENAL OF THE ROYAL HORTICULTURAL SOCIETY. 



to which they are subject has on their development. Schimper gives 

 the following summary of the result of scientific observation on the 

 subject: " The action of light on plants is either invigorating or 

 restricting, creative or destructive, according to its intensity, and ac- 

 cording 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 the subject. 



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

 is wholly excluded. Even light of 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 illumination retards, and eventually arrests the 



B A 



Fig. 50. — Germander (a) exposed to a low temperature during the night 



AND OUTSIDE IN FULL SUN DURING THE DAY, AND THE SAME SPECIES (b) 

 cultivated UNDER NATURAL CONDITIONS IN THE CLIMATE OF PaRIS. {After 



Bonnier.) 



process. The optimum intensity of light for growth in thickness of 

 leaves is considerably higher than for the growth of leaf surface, so that 

 strongly illuminated leaves are small and thick."* In other words, a 

 brilliant illumination exercises a dwarfing influence upon plant life. 

 The length of the axes and internodes is reduced, the leaves decrease in 

 area and increase in thickness. Light of an extreme brilliancy can even 

 possess a destructive influence by the decomposition of chlorophyll. I 

 give the statement by Schimper as representing the result of the general 

 experience of qualified observers. It is, of course, callable of amplifica- 

 tion, but I do not think that the research and experiments of more 

 modern authorities will afford evidence to show that as a summary of 

 existing knowledge concerning the influence of light upon vegetation, 

 Schimper's statement is in any respect inaccurate. f The difficulty 



* A. F. W. Schimper, Plant Geography, English Edition, p. 57 



t Leclerc du Sablon, Traite de Physiologie vcgetale, pp. 170-171, 434-449. 



