96 GROWTH OF PLANTS 



generally in buds of trees and shrubs of the temperate zone. The buds go 

 into the mnter in the dormant condition and are after-ripened by the 

 cold weather so they grow with vigor in the spring. Consider one example, 

 the peach tree. Its leaf buds are after-ripened by the cold weather of 

 winter and consequently take on vigorous growth in the spring. If one 

 goes into Georgia he will note that the peach trees are smaller because of 

 insufficient after-ripening of the buds during the short winters. If one 

 goes far enough south, the peach no longer even persists, probably because 

 there is not sufficient winter to after-ripen the buds even partially. The 

 buds of trees of low-altitude tropical plants do not require low tempera- 

 tures to after-ripen the buds and give them growth vigor. Perhaps one 

 could select strains of peaches that had this need to a less degree and 

 extend the culture farther south. 



It is evident that the epicotyls of many seeds are not dormant in the 

 sense we have just described and consequently do not need low- temperature 

 after-ripening. However, we shall later discuss classes of seeds having 

 dormant epicotyls that need low-temperature after-ripening but that do 

 not have a sluggish growth in other parts of the embryo. In classes of 

 seeds to be described later we shall frequently meet dormant embryos 

 that require low-temperature stratification for after-ripening. 



Davis and Rose " believed that the radical is the dormant organ of the 

 Crataegus embryo, but Flemion ^^ has shown that the epicotyl is the dor- 

 mant organ in this embryo. The radical is sluggish in the early growth of 

 the embryo, but once started, it grows A\dth vigor. On the other hand, 

 the eipcotyl shows a persistent dwarfishness in growth until it is after- 

 ripened by a period of low-temperature exposure. This could be discovered 

 only by forcing dormant embryos to form seedlings without low tempera- 

 tures and by continuing the growth of these seedlings over long periods. 



Very recently Flemion and Waterbury ^'^^ have thrown some additional 

 light on the persistence and nature of the dwarfishness in seedlings grown 

 from dormant embryos. After-ripened peach embryos were deprived of 

 all or part of their storage material. Death resulted when both cotyledons 

 were removed at the time of planting; but when they were removed ten 

 days later, small but normal seedlings were obtained. When various parts 

 of the cotyledons were removed at the time of planting, normal plants 

 resulted, except that the plants were smaller when only one-third of one 

 cotyledon remained. However, none of these small plants had the tele- 

 scoping of internodes or other dwarfing characteristics so typical of seed- 

 lings obtained from non-after-ripened embryos. Dwarfish seedlings have 

 apparently adequate root systems, for in dry weight determinations the 

 ratio of root to top was always greater in the dwarfs. When the growing 

 tip of a normal seedling was grafted on the stem of a dwarfish seedling, 

 the result was a normal seedling, showing that the root system of the dwarf 

 was capable of sustaining normal growth and that apparently there was 

 no substance in the root which inhibited shoot growth. Interposing by 



