[Chap. XXXIV GROWTH, DORMANCY, GERMINATION OF SEEDS 395 



ing months the embryo grows and its three parts become diflFerentiated. 

 Germination of the seed occurs after the embryo has fully developed in 

 the seed. To this class of seeds with only rudimentary embryos belong 

 those of buttercup, anemone, adder's tongue, holly, columbine, and 

 hepatica. Some of these seeds germinate more rapidly in dilute sugar 

 solutions than in water. The dormant period of these seeds is a time of 

 active chemical and morphological development of the embryo. The 

 embryo itself is not dormant. 



A second group of plants with fully formed embryos have seeds that 

 are also truly dormant when shed. These seeds undergo a process of 

 after-ripening in which certain chemical changes in the embryo slowly 

 occur. They do not germinate at once when planted under external con- 

 ditions ordinarily suitable for growth; but if the seed coats are removed 

 and the embryos are treated with a dilute acid, some of them germinate 

 comparatively soon. The dormancy of these embryos is conditioned by 

 chemical processes. 



Little is known about the processes that occur during dormancy, but 

 it is wrong to assume that all dormant seeds contain a "resting embryo." 

 Experiments have shown that during after-ripening of this last class of 

 seeds the embryos become more acid, enzymes are more abundant, com- 

 plex carbohydrates and proteins are hydrolyzed, and respiration is in- 

 creased. These are merely the superficial, easily tested indicators of 

 more important, but unknown, steps in the reorganization of the proto- 

 plasmic system. 



To this group of seeds with embrvos in which an increase in acidity 

 appears to be a primary part of the process of after-ripening belong 

 those of apple, rose, cherry, sugar maple, giant ragweed, basswood, 

 cotoneaster, peach, and plum. 



After-ripening may be hastened in many of these seeds by storing them 

 at a temperature of 40° F. Other seeds after-ripen soonest at a tempera- 

 ture nearer 50° F., and still others after-ripen best with daily fluctuations 

 in temperature as low as 40° F. and as high as 70° F. Moist acid peat 

 probably is the best medium in which they may be placed during the 

 dormant period. Table 13 is a summary of some of the results of after- 

 ripening tests made at the Boyce Thompson Institute for Plant Research. 

 Most of these seeds were stored dry until February, and were then 

 stratified, or mixed with moist granular peat, and placed at a variety of 

 temperatures. 



