DORMANCY IN SEEDS 



109 



available nitrogen and are often limited in the decomposition by lack of 

 available nitrogen. We have already learned that nitrates and other nitro- 

 gen compoimds substitute for light in light-favored seeds. Gassner ^- 

 found that nitrates substitute for light in forcing the germination of 

 Chloris ciliata seeds, although the enveloping tissues are not permeable to 

 nitrates. This raises the question as to what extent nitrates favor the 

 germination of light-favored seeds in darkness by furthering the decom- 

 position of the coats by organisms. 



Table 12. Percentage Germination of Symphoricarpos orhiciilafvs Seeds When Mixed 

 in Moist Peat Moss and Kept at Various Temperatures.* 



Temp. 

 (°C) 



1 



5 

 10 

 15 

 20 



Percentage germination 



0.5 year 







1 



5 

 

 



5 years 



3 



10 



74 



5 







* Duplicate lots of 200 seeds of 1934 crop; experiment started March 6, 1935. 



We have seen that the coats of Symphoricarpos seeds are decomposed 

 by organisms most rapidly in the soil at relatively high temperatures, and 

 that the embryos after-ripen best at about 5° C (41° F). Can an inter- 

 mediate constant temperature be selected that will permit both processes 

 to occur in succession, followed by germination? Table 12 shows the ger- 

 mination of seeds of S. orbiculatus at constant temperatures, 1°, 5°, 10°, 

 15°, and 20° C (34°, 41°, 50°, 59°, and 68° F). Only 10° C (50° F) gave 

 any considerable germination, amounting to 43 per cent after one year 

 and finally rising to 74 per cent after five years. At 5° C (41° F) there was 

 only 10 per cent germination after five years and at 15° C (59° F) only 

 5 per cent after five years. Temperatures much below 10° C (50° F) per- 

 mit only very slow dismtegration of the coats, and temperatures much 

 above 10° C (50° F) do not lead to after-ripening of the embryos. 



Flemion "^^ has found that seeds of various species of Crataegus differ 

 markedly in their requirements for germination. C. cordata [Phaenopynim] 

 and C. coccinea belong to the class previously discussed. They need two 

 and one-half to five months' low-temperature stratification to after-ripen. 

 If planted outside in the fall, they after-ripen during the ^vinter and pro- 

 duce seedlings the following spring. They are not two-year seeds. The 

 seeds of C. fiava, C. punctata, C. Crus-galli, and C. rotundifolia are two-year 

 seeds. Fall planting will not produce seedlings the first spring, but seed- 

 lings are produced the second spring after fall planting. This gives a 

 summer in the soil for overcoming coat resistance. Spring planting of 

 these produces seedlings the next spring. This gives a summer in the soil 



