132 THE BIOLOGY OF FLOWERING PLANTS 



light conditions, and that in this, and not in any marked 

 degree of plasticity or specialisation, lies the reason of its 

 extraordinary success. It is a very remarkable thing that, 

 while in every other respect the organisation of the plant 

 shows the widest sort of variation, in this one point, of 

 pigmentation, there is extraordinarily little change through- 

 out the vegetable kingdom. 



This interpretation of the utility of the green colour 

 of chlorophyll is supported when we consider that there is 

 every reason to believe that the pigment complex we know 

 to-day has persisted throughout the history of the plant 

 kingdom from its origin. It has been a successful feature 

 of plant-life under general conditions of illumination 

 different from those of the present day ; for it is likely 

 that in former epochs an atmosphere much more nearly 

 saturated with water and much cloudier than that of 

 to-day, made diffuse light the normal kind of illumina- 

 tion. This again emphasises the importance of diffuse 

 light. We m_ay look on chlorophyll as one of the most 

 successful and most conservative of the products of life, 

 with relations so generalised that a very close fit to any 

 particular set of conditions is not to be expected. 



§ 15. Chlorophyll in its Relation to Assimilation 



The relative amounts of the four chlorophyll pigments 

 is fairly constant throughout the flowering plants, but the 

 total chlorophyll content is subject to considerable variation. 

 In the life-history of an individual leaf it, of course, changes 

 in a definite and well-known fashion. The young leaf 

 contains relatively little chlorophyll, although it may con- 

 tain much yellow pigment ; on exposure to light the 

 amount of chlorophyll increases to a maximum which, as 

 Wiesner has shown, occurs about the time the leaf attains 

 its full size. This is the case at least for deciduous leaves ; 

 evergreen leaves may not attain their full colour till the 

 second year of their life, or later. After this maximum is 



