24 PLANT LIFE 



as regards the direction of the light is, however, 

 a very widespread one, and we shall meet 

 with it again, although in a modified form, in 

 studying the behaviour of the highest plants ; 

 for the property of irritability which in 

 Chlamydomonas finds expression in the inde- 

 pendent movement of the organism as a whole, 

 is a necessary condition of existence for every 

 plant to a greater or less extent. Only in 

 this way is it possible for it to place itself 

 en rapport with a variable and changing 

 environment, and hence with the physical 

 conditions under which it lives. 



A brief survey of the more salient physical 

 characters of chlorophyll will not be out of 

 place here, inasmuch as they stand in sugges- 

 tive relation to the properties of this remark- 

 able substance. 



It is easy to extract the green colour of plants 

 by soaking a quantity of grass in strong 

 alcohol. A dark green liquid will thus be 

 obtained which if examined by reflected light 

 will appear to be not green, but blood-red in 

 colour. This property of " fluorescence " 

 is not confined to chlorophyll, but is shared 

 by many other organic and some inorganic 

 substances, and it affords useful hints as to 

 their more intimate chemical architecture. A 

 solution, prepared by the rough-and-ready 

 method indicated above, is of course not pure 

 chlorophyll, but it contains several other 

 colouring matters which can be separated from 

 it by appropriate means. 



