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. 
