92 



GROWTH OF PLANTS 



bryos varies considerably with the variety and species. Apple, pear, most 

 Crataegus species, Prunus americana, and others behave much like the 

 embryo of Sorbus shown in Fig. 26, in which only the cotyledon in con- 

 tact with water grows; also in some of these the epicotyl elongates with- 

 out any growth of the radical and hypocotyl (Fig. 30) when the seeds are 

 placed on moist filter paper. The dormant peach and Rhodotypos embryos 



Figure 30. Excised non-after-ripened apple embryos grown in Petri dishes 14 days 

 at room temperature. 



are less sluggish than the ones mentioned above. The latter in the main 

 show growth of both cotyledons and the hypocotyl and radical when 

 placed on moist filter paper, but even in these the growth of the non- 

 after-ripened embryo is very much slower than that of the after-ripened 

 embryo. 



Sorhus embryos attain sufficient after-ripening after six weeks' stratifi- 

 cation at 1° C (34° F) in intact seeds to give vigorous early gro^vth, as 

 shown by their development when excised; but 12 weeks' stratification is 

 needed for the intact seed to give full germination. Very recent unpub- 

 lished work by Flemion indicates that the dwarfishness in the later growth 

 of the embryo is not entirely overcome until the intact seeds have been 

 stratified long enough to enable them to germinate completely. Seeds 

 with dormant embryos will finally germinate at the optimum stratification 

 temperatures, but they will do so more quickly if they are transferred to 

 a somewhat higher temperature after complete after-ripening. Transferring 

 partially after-ripened seeds to germinators at 20° C (68° F) is likely to 

 throw them into secondary dormancy, and even fully after-ripened seeds 

 do better if planted at relatively low temperatures such as are met in 

 early spring planting. Indeed, Pack ^^ found that various species of after- 

 ripened Juniperus seeds germinated faster at the best after-ripening tern- 



