INDIVIDUAL AND SPECIFIC PECULIARITIES 359 



solely dependent upon the amount of chlorophyll present 1 , the maximal 

 power of assimilating carbon dioxide being acquired when the leaves are 

 from two-thirds to full grown. 



It is not yet certain whether in shade- loving (heliophobic) plants the 

 general cell-protoplasm may not be more sensitive to light than the chloro- 

 plastids are. In all the photophilic and heliophobic plants examined, the 

 chloroplastids seem to be affected sooner and more markedly than the 

 general cytoplasm, whereas in Hydra viridis it appears that the animal 

 protoplasm is more sensitive to the action of light than are the symbiotic 

 algal cells it contains, and it is not impossible that in many shade plants 2 

 the general cytoplasm may be more sensitive than the chloroplastids are. 

 The various protective adaptations against intense light, such as are due to 

 the presence of absorptive pigments, to movements of the leaves or chloro- 

 plastids, to structural arrangements, &c., may be of importance to protect 

 not only the chloroplastids but also the general cell-plasma from over- 

 exposure, for as a matter of fact many colourless protoplasts are extremely 

 sensitive to light and readily injured by intense illumination. The fact that 

 the epidermis of terrestrial plants is usually without chlorophyll, that the 

 chlorophyllous guard-cells of the stomata are usually on the under surface 

 of bifacial leaves, and that the chloroplastids when subjected to intense 

 illumination assume positions in which they are least exposed, all indicate 

 the necessity of protecting the latter from light of excessive intensity 3 . The 

 practical utility of these phenomena does not concern us, but it must be- 

 remembered that these protective adaptations need not necessarily be of 

 importance only to the function which they more especially subserve. 



Although protective adaptations exist against excessive exposure, 

 the general arrangement and distribution of the chloroplastids is such that 

 they receive as much light as possible. This is clearly shown in the 

 structure and shape of all chlorophyllous organs, and the arrangements 

 of the leaf-cells and of the chloroplastids is apparently such as to permit 

 the penetration of a maximal amount of light to the more deeply situated 

 green layers. It is easy to see that the more evenly the light is dis- 

 tributed, the more nearly may an optimal illumination be reached for the 

 deeper layers without the peripherally situated chloroplastids being exposed 

 to an injurious intensity of light. The presence of non-chlorophyllous areas, 

 and the refraction and reflection which occur in every leaf, may serve to 



1 Corenwinder, Ann. d. chim. et d. phys., 1858, iii. ser., T. LIV, p. 330; Ewart, I.e., p. 452. 

 On the influence of the temperature, cf. Sect. 58. 



a See Ewart, I.e., pp. 439, 573; Annals of Botany, 1897, Vol. xr, p. 442 ; 1898, p. 387 ; and 

 Sect. 58. 



3 The movements dependent on light will be discussed later. On pigments and their im- 

 portance, cf. Sect. 88. See also Ewart, Annals of Botany, Vol. xi, p. 475. 



