78 DUDLEY MEMORIAL VOLUME 



simple terms I believe the experiments which are described in the preceding 

 pages show is not the case. Light does not always check growth, other things 

 being equal. Instead, it promotes it, also, other things being equal. 



When plants are grown in darkness, their leaves are small and may even 

 be misshapen ; it is their stems which are long, and they lack thickness. The 

 organs devoted to food manufacture require light for their normal develop- 

 ment. This, too, has long been known and it has been explained on the 

 supposition that use (activity) and food are necessary for the development 

 of leaves and their tissues. Given a certain amount of light, growth (and 

 development) should take place at a certain rate and to a certain amount, 

 other things being equal. By a certain amount of light is meant that quan- 

 tity which the plant or organ can use or which so penetrates its living cells 

 as to affect them. We conceive that, under usual conditions, a plant and its 

 organs grow in direction, rate and amount into such positions as afEord what 

 may be called the optimum, all its activities contributing to this resultant. If 

 this be true, the position and size of a plant represent the influence of cir- 

 cumstances upon its substance. By modifying the circumstances in any 

 way, we may also influence the resultant. Without increasing the quantity 

 of light available or changing the quality, and without, so far as I can se^ 

 affecting the supply of carbon dioxide and of other food materials, the plants 

 used in the experiments here described exhibit remarkable differences accord- 

 ing to their different exposures to light. The usual position occupied in the 

 light by chlorophyll-containing organs is that which presents the greatest 

 possible contrast between the amounts of light available on the two sides of 

 these organs. The positions of ordinary leaves show this, and I am not sure 

 that even such vertical and two-faced leaves as those of Eucalyptus do not also 

 show this more or less. But in darkness and on turn-table there is no such 

 position of contrast. There is no contrast. In darkness — that is, under con- 

 ditions free from the alleged depressing influence of light upon growth — 

 growth does not take place to the usual extent in chlorophyllrcontaining or- 

 gans, even when food is supplied in adequate quantities and in suitable form. 

 Under these conditions growth should be greater. But one may attribute this 

 lesser growth to lack of use of the chlorophyll apparatus. This hypothesis 

 may usually be correct without necessarily being complete. For light may con- 

 ceivably stimulate, apart from its effect on food manufacture and the appa- 

 ratus concerned. The manufacture of food depends upon many factors, of 

 which sufficient water, light, carbon dioxide and chlorophyll are the most 



