262 LECTURE XIII. 



processes. But, as we have just learned, there is no evidence 

 to shew that light has any such influence on destructive 

 metabolism, still less is there any evidence to shew that the 

 rays which are absorbed by chlorophyll are especially active 

 in this respect. 



The most obvious effect of light upon the metabolism of 

 plants is its influence in causing the formation of colouring- 

 matters. The most conspicuous example of this is afforded 

 by chlorophyll. Chlorophyll is usually not formed in the 

 absence of light (see p. 239) ; I say, usually, for, according to 

 Sachs, chlorophyll is formed in the cotyledons of some 

 Conifers and in the leaves of Ferns in complete darkness, 

 provided that the temperature is sufficiently high. If a seed 

 of a green plant be made to germinate in the dark, the seed- 

 ling will present, amongst others, this peculiarity, that it is 

 not green, but is of a pale yellow colour, or it may be quite 

 white. Such a plant is said to be etiolated. The yellow 

 colour of an etiolated plant is due to the presence of a yellow 

 colouring-matter, etiolin, in its corpuscles ; the white colour 

 may be due to the absence of any colouring-matter, but 

 it appears probable that the corpuscles contain a colourless 

 chromogen, termed by Sachs, leucophyll, which under appro- 

 priate conditions gives rise to chlorophyll. 



The formation of chlorophyll will take place in light of very 

 low intensity, but still, as Wiesner's experiments shew, there 

 is a lower limit of intensity below which light is inactive. 

 It is true that a great many observers agree in stating that 

 the requisite degree of intensity is different for different 

 plants, but this difference must not be taken to mean that 

 the chemical process of the formation of chlorophyll can go 

 on at a low intensity of light in some plants and only at a 

 relatively high intensity in others : it is due, as Wiesner 

 points out, to the fact that other conditions are not the same 

 in all cases ; for instance, leaves differ in the thickness of 

 their epidermis, in the presence or absence of hairs, in their 

 mode of vernation, etc., and in proportion as the access of 

 light to the mesophyll cells is interfered with by any of these 

 structural peculiarities the formation of chlorophyll will be 



