n DEVELOPMENT AND ENVIRONMENT 31 



Meanwhile, we must not forget that " the night- 

 temperature is commonly recorded as a minimum only, 

 and that the true mean night-temperature may be some- 

 what different. Moreover, the internal temperature of the 

 tissues is appreciably affected by clouds, which check 

 radiation. 



Having now discussed the main factors which control 

 growth during the first half of the season, we can turn to 

 the examination of the water factor in more detail, since 

 its importance will be considerable at a later date. Before 

 so doing, an example may be studied which will remind 

 us that the idiosyncrasies of the plant have to be taken as 

 the base-line for all environmental effects. 



Length of the seedling stem. — That portion of 

 the stem known as the hypocotyl, situated between the 

 true root and the seed leaves, forms the greater part of 

 the " root " which emerges from the seed-coats on 

 germination. It is distinguished from the true root by 

 the black speckling of resin glands. The length to which 

 it attains after raising the cotyledons out of the soil 

 would appear to be a matter of pure chance. Seed buried 

 under clods of earth, or deeply sown, develops a stouter 

 hypocotyl in reaction against the mechanical resistance ; 

 much of the reserve food having been exhausted in the 

 effort, the aerial portion of the hypocotyl is shorter, and 

 the cotyledons smaller. 



A series of seedlings grown under uniform conditions 

 showed that specific foundations were discernible in such 

 a trivial character even as this. The strains employed 

 were derived from an American Upland (King), and an 

 Egyptian (Sultani). Having been soaked for a night at 

 30° C. they were planted at a uniform depth of 2 cm., at 

 uniform distances of 5 cm., in finely sifted soil which had 

 been uniformly damped, and the hole which had been 

 dibbled for each seed was filled by more fine soil without 

 compression. The box containing the soil was kept in 



