428 PLANT PHYSIOLOGY 



are retarded in their growth, at least in the case of short-day 

 plants. Seeds that have already passed through the light stage, 

 when afterward sown under conditions of a long day, very soon 

 bloom at any day length, even when given uninterrupted illumi- 

 nation. Unfortunately as yet it has not been possible to make 

 all plants pass through the light stage in the form of germinating 

 seeds, and this greatly limits the practical appHcation of photo- 

 periodism. The increase of the length of day by means of 

 electric Hght is very limited in apphcation being useful chiefly 

 for greenhouse crops. 



98. Physiological Differences between Winter and Spring 

 Types, Early and Late Plants. Basis of Selection for Earliness. 

 The discovery of yarovisation and of photoperiodism has 

 changed completely the concept of early- and late-ripening plants. 

 Before these discoveries, it was usually assumed that the length of 

 the vegetative period of one variety or the other is the result of the 

 inherent speed of its development, independent of the environ- 

 mental conditions. It was known that the rate of development 

 depends upon the temperature and that in hotter climates 

 development is more rapid than in colder regions; but the 

 sequence of earliness of the varieties was assumed to be constant. 



At present, questions of earliness are approached in a quite 

 different way. In the discussion of photoperiodism, it has 

 already been emphasized that different varieties react differently 

 to the length of day. Let us examine, for instance, the Mandarin 

 and Biloxi varieties of soybeans, both typical short-day plants. 

 With comparatively short day, both varieties mature simul- 

 taneously, but Biloxi is more susceptible to an increase in day 

 length.. When grown in the north, it is a later variety than 

 Mandarin. In more southern regions, on the contrary, Biloxi 

 will develop in advance of the Mandarin variety. Thus, earli- 

 ness is only relative, depending upon the character of the reaction 

 of varieties to changes in the environmental conditions. It 

 must be noted that different phases of development depend to 

 various degrees upon the environmental conditiens. The length 

 of day influences chiefly the duration of the period between the 

 seedhng stage and blooming, while the length of the period 

 between flowering and ripening of the seeds depends chiefly on 

 the temperature. That is why varieties blooming early but 

 ripening slowly will react differently to changes in environmental 



