194 GROWTH 



expansion of the leaf. If a shortage of carbohydrate leads to 

 a fall in the growth rate of the root, a fall in the relative growth 

 rate of the leaf would follow owing to an increasing dispropor- 

 tion of available nitrogen. 



From his own observations, coupled with those of Priestley * 

 and others, Gregory favours the conception of a growth 

 stimulating photochemical reaction, independent of carbon 

 assimilation, acting on a nitrogenous substrate. 



LIGHT. 



The influence of light on growth is a subject of consider- 

 able magnitude, especially when the term growth is used in 

 its general sense : the directive action of light in tropistic 

 and kindred phenomena, its influence in the determination 

 of growth form and the facies of a flora are aspects of the 

 subject outside the scope of the present consideration. 



The action of light on growth is both direct and indirect, 

 and its action is most marked in those members which use 

 light as a source of energy. Thus for the ordinary green plant, 

 increase without light is an impossibility since light is the 

 source of energy for the making of food ; in non-green mem- 

 bers of plants, in total parasites, and saprophytes, on the 

 other hand, light, for obvious reasons, is not a factor of con- 

 sequence. It is a laboratory commonplace to find that for 

 the subjects generally used for experimental purposes, light 

 influences growth, as indicated by its external expression of 

 increase in length and area, in various ways. Thus internodes 

 in darkness attain a much greater length than in light ; leaves 

 may develop hardly at all in darkness, as in the pea, whilst in 

 other plants, the wheat for example, the leaves attain a size 

 more or less equal to those grown in light. This difference in 

 behaviour is possibly due to the amount of carbohydrate, rela- 

 tively large in the wheat and relatively small in the pea, avail- 

 able for structural purposes. f To take a specific instance, 



* Priestley : " New Phyt.," 1925, 24, 271. 



| The work of Priestley (" New Phyt.," 1925, 24, 271) indicates that 

 very special precautions must be taken to exclude light and thus to see the 

 true etiolated condition. As will be seen later, a very small exposure to 

 light may have a morphological effect. 



