340 Morphogenetic Factors 



of flowering in some of the cereal grains so that "winter" varieties, 

 superior in certain respects, can be treated like "spring" varieties. A win- 

 ter race of rye, for example, is normally sown in the fall and flowers and 

 fruits the next season. If planted in the spring it will produce abundant 

 vegetative growth but no flowers. A spring race sown in the spring will 

 fruit in that growing season. If seed of winter rye is soaked in water, 

 however, and is then exposed to low temperature (0 to 10° C or there- 

 abouts) for a few hours or days, it can be sown in the spring and will 

 bear flowers and fruit just as rapidly as spring rye does. Grain germi- 

 nated at 1°C and then planted will produce rye that flowers in 68 days, 

 but if germinated at 18 °C, flowering does not take place for 150 days 

 (Gregory and Purvis, 1938). The vernalized seed may be kept dormant 

 for some time or even dried and it will still grow like spring rye when 

 it is planted. 



These facts are explained on the assumption that development is to a 

 great extent independent of growth, that in an annual seed plant there 

 is a specific series of developmental stages, each a necessary precursor 

 to the next, and that these stages require for their completion different 

 environmental conditions, especially as to temperature and light. This 

 is an aspect of the general concept of phasic development previously 

 discussed (p. 205). 



In these cereals the first stage is the one in which the floral initials 

 are formed, and for this process low temperatures are necessary. Winter 

 rye sown in the fall will produce these initials because it is exposed to 

 the low temperatures of winter but if it is sown in the spring the tem- 

 perature is not low enough for this to happen. Vernalization thus acts as 

 a substitute for winter temperature. 



The next stage, the development of the flowers themselves, usually 

 requires higher temperatures but also relatively long days. Flowering 

 will be delayed indefinitely if plants at this stage are exposed to short 

 light periods. In the long, warm days of spring, both winter and spring 

 varieties will thus come into flower. The difference between them is that 

 spring varieties will form floral primordia in these warm days and winter 

 varieties will not do so without treatment. 



In certain plants a definite number of primordial structures are formed 

 at the growing point of the embryo, and it can be shown that the fate of 

 some of these has not been determined in the seed but that leaves or flow- 

 ers will form from them, depending on environmental conditions. Thus 

 in winter rye there are usually 25 embryonic primordia. The first seven 

 will always develop into leaves. The next 18 are indeterminate, and the 

 lower the temperature to which they are exposed, the more of them will 

 develop flowers. At high spring temperatures, none of them will do so 

 unless previously chilled. 



