ADAPTATIONS 253 



forms of adaptation. Furthermore, it is difficult to conceive the 

 general application of this idea to alterations of habits of growth 

 of plants, since a plant cannot have any such thing as a volun- 

 tary control over the amount of " use " which it makes of its dif- 

 ferent organs in response to changes of environment. The com- 

 mon form of statement that a plant develops an organ, or a process 

 to meet a certain need, or modifies its habits of growth to meet a 

 change of environment are, of course, purely metaphorical, and 

 can only be taken to mean that such processes are mechanical 

 responses to changes in external conditions. 



The nature of the mechanism by which these responses are 

 accomplished is, as yet, wholly unknown. There is accumulating 

 a large mass of experimental evidence which goes to show that, 

 while both temperature and light are very important factors in 

 determining the type of changes which will take place in a living 

 organism, the so-called " photochemical action of light " is by far 

 the most potent of all the climatic factors which influence the 

 course of development of a plant. But we have, as yet, no inkling 

 of how the protoplasm of the plant adjusts or controls its responses 

 to variations in any of these external factors. 



With these general considerations in mind, we may now pro- 

 ceed to the consideration of certain particular types of adaptations. 



CHROMATIC ADAPTATIONS 



Adaptations have been observed in both the energy-absorbing 

 pigments of the general tissues and in the ornamental epidermis 

 pigments of plants. The former are by far the most important 

 from the physiological point of view; while the latter may have 

 interesting biological significance. 



Under nearly all conditions of growth of land plants, the supply 

 of the chlorophylls and their associated pigments provides for 

 the absorption of solar energy far in excess of the amount 

 necessary for the photosynthetic assimilation of all the carbon 

 dioxide which is available to the plant. It has been shown that 

 an active green leaf, on an August day, can absorb eight times as 

 much radiant energy as would be required to assimilate all the 

 carbon dioxide present in the air over its surface. No land plant, 

 under normal conditions, develops suppplementary pigments in 



