ADAPTATION 197 



who wished to enter more deeply into these problems. We 

 can only mention here that permeability to water itself 

 also plays its part, and that, according to Overton's experi- 

 ments, it is a kind of solubility of the media in the very 

 substance of the cell surface on which all permeability and 

 its regulation depend. 



Chromatic Regulations in Algae. The phenomena of 

 osmotic pressure and its regulation may be said to be the 

 preliminaries of metabolism proper, conditions necessary for 

 it to take place. Now there is another branch of such 

 preliminaries to metabolism, in which the most interesting 

 regulation phenomena have been lately discovered. It is 

 well known that what is called assimilation in plants, that 

 is, the formation of organic compounds out of carbon dioxide 

 (C0 ) and water, occurs only in the light by means of 

 certain pigments. This pigment is in all higher plants and 

 in many others the green chlorophyll, but it may be different 

 in certain species of algae, and can generally be said ] to be 

 of the colour complementary to the colour of those rays 

 which especially are to be absorbed and to be used for 

 assimilation. But here we have " adaptedness," not 

 adaptation. It was in some species of primitive algae, the 

 Oscillariae, that Gaidukow 2 found a very interesting instance 

 of an active regulation in the formation of pigments. These 

 algae always assume a colour which corresponds to the 

 accidental colour of the rays of the medium and is com- 

 plementary to it ; they become green in red light, yellow in 

 blue light, and so on that is, they always actively take that 

 sort of colouring which is the most suitable to the actual 



1 See Stahl, Naturw. Wochenschrift, N. F. 5, 1906, No. 19. 

 2 Arch. Anat. Phys., Phys. Abt. Suppl., 1902. 



