88 



GROWTH OF PLANTS 



excising the embryo and placing it on moist filter paper. The non-after- 

 ripened embryo showed growth only in the cotyledon lying against the 

 moist filter paper after 21 days, while the after-ripened embryo showed 

 some enlargement of both cotyledons and a spreading apart of the coty- 

 ledons and an elongation of several centimeters of the hypocotyl after only 

 three days. One is struck by the enormous increase in growth vigor of 

 this embryo induced by six weeks of low-temperature stratification. In 

 this same figure, one seed that had four weeks' stratification partially 

 removed the dormancy since there is some growth after three days. There 

 is no growth in the embryo from a seed stratified only two weeks. 



Figure 28. The degree of after-ripening attained by seeds of Sorbus aucuparia which 

 had been stratified for two months at various temperatures, as shown by the changes 

 in the excised embryos after being on moist filter paper for two days. 



Correlation in the growth of the several organs of the dormant embryo 

 is changed by after-ripening at low temperatures. In after-ripened embryos 

 and embryos of non-dormant seeds the hypocotyl and radical grow first, 

 and a root is formed with abundant root hairs. Later the cotyledons begin 

 to grow and spread apart, and still later the epicotyl develops. In dormant 

 embryos not only is the growth much slower but the cotyledons grow 

 first — often only one of them if it alone is in contact ^vith water; also, 

 the epicotyl often elongates before the hypocotyl and radical. Fig. 28 

 shows the effectiveness of various low temperatures in after-ripening the 

 embryos of intact Sorhus seeds in tw^o months: 1° C (34° F) gives greatest 

 vigor, 5° C (41° F) next, while -5° C (23° F) and weekly alternations at 

 -5° and +5° C (23° and 41° F) are ineffective. It is evident that tem- 

 peratures above freezing are effective, whereas freezing temperatures and 



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