SPECIFIC GROWTH FACTORS 333 



(Brotherton and Bartless, 1918), with the result that in the light 

 growth is due mostly to an increase in size of cells rather than in 

 their number. Also light causes the stomata to open and favors 

 increased transpiration, thus decreasing turgor, one of the essen- 

 tials for good growth. 



Plants grown in the dark, — so-called etiolated forms, — are ex- 

 tremely thin and spindling. They grow rapidly until all the food 

 stored in the seed is consumed, making a really remarkable growth, 

 considering the small amount of dry matter which they have at 

 their disposal. Certain observers have interpreted this rapid, 

 spindling growth as an adaptation. To them, the plant is searching 

 for light and, if it grows tall and long, it will have a better chance 

 to find it; but it is not necessary to inject such ideas of purposeful- 

 ness into the explanation. The reason for the excessive growth 

 may be found not in the presence of darkness but rather in the 

 absence of the checking factors accompanying light. 



It is a matter of common observation that corn and most plants 

 grow best at night, although the change in temperature and 

 humidity also play an important part in the increased growth. 

 Forest leaves are larger in the shade ; and cacti in the bright light 

 are smaller and stockier than those grown in the shade. In the 

 growth of many garden vegetables such as lettuce and cauliflowers, 

 the " half-shade" system of propagation is. used, in which the 

 growing plants are cultivated in the shade of cheesecloth placed 

 at a suitable distance from the ground. This cuts down the light, 

 with the result that the plants are more succulent, more tender, 

 with milder odors and flavors, and thus more desirable in a fancy 



market. 



Tobacco is also commonly grown under a half-shade system. 

 When the light is thus reduced 20-40%, a much higher grade of 

 leaf is produced, about twice normal size, and with a much finer 

 texture. 



While all these effects are also connected with changes in tran- 

 spiration as well as with the amount of moisture in the soil, the 

 temperature, and the biological relations with other organisms 

 (such as the fungi) of the soil, there can be no doubt that a part 

 of the effect at least is due to light. That this is true, has been 

 corroborated by laboratory experiments in which these other 

 factors were kept constant. 



Strong light, especially that from the blue end of the spectrum. 



