SPECIFIC GROWTH FACTORS 335 



below 12 hours decreased growth, and with light alternations of 

 1 minute growth was at a minimum. Such alternations seemed to 

 have almost the effect of continuous darkness and the plants were 

 chlorotic. On the other hand, when the alternating light and dark 

 periods were reduced to 15 seconds in length, growth increased, 

 and extremely short alternations (5 seconds) had the general 

 effect of continuous illumination. 



Light thus exerts a double influence upon growth; it determines 

 the rate at which the food is manufactured and also the rate at 

 which it is used. An intensity which favors one may hinder the 

 other, and much careful research remains to be done to determine 

 the optimum combination of factors for each species of plant. 



Light affects the germination of seeds in various ways according 

 to the species. Some, like the grape, pine, and mistletoe, are 

 favored in germination by light, while others, such as the jimson 

 weed, germinate best in the dark. Still others such as chicory, 

 parsnip, and parsley are unaffected by the light. It is thought 

 that light aids those of the first group by hastening ripening 

 processes which must be completed before germination can occur; 

 and some seeds which ordinarily require light will germinate in 

 the dark if iron salts, organic acids, alcohol, or other stimulants 

 are supplied. As the temperature falls, the optimum amount of 

 light increases, which indicates that the light acts partly through 

 its heat or its energizing property. 



The morphogenic effect of light is very pronounced. In the 

 Bryophytes and Pteridophytes, light plays an important role in 

 determining the early development of the plant. In Equisetum 

 the first cross wall of the spore is formed at right angles to the 

 incident light. In Marchantia gemmae, the rhizoids always form 

 on the shady side, and in fern prothallia the formation of the 

 sexual organs on the ventral side^ is largely determined by light. 

 Among the higher plants, the effect of light is plainly seen in the 

 structure of the leaves. Plants growing in very bright light 

 have several layers of palisade cells, while those in the shade have 

 only one such layer. Similarly, erect leaves like the Iris have 

 palisade layers on both sides. In the bluebell (Campanula rotundi- 

 folia), the leaves at the base of the plant in the shade are cordate 

 with long petioles, while those along the stem in the light are linear 

 with short petioles. If a plant with both kinds of leaves is grown 

 in very dim light, the buds on the upper stem, which would or- 



