Epicotyl dormancy

Low temperatures are involved in quite different processes. On the one hand they can lift barriers imposed by inhibitors and thus render the embryo capable of germinating. This particular temperature effect should actually have been included in the previous section. On the other hand germination itself can, in many cases, still take place. However, the subsequent development of the seedling is inhibited if it is not subject to lower temperatures. So-called physiological dwarfs then develop. Considered in more detail, the growth of both the epicotyl and the hypocotyl or simply the epicotyl can be inhibited. Accordingly, one speaks of either hypocotyl and/or epicotyl dormancy which must be broken by lower temperatures. In this case dormancy is really the wrong word since it is actually retarded development. 

An interesting mechanism is found among certain species of the Liliaceae genus Trillium (Fig. 205) among others. Here cold is needed twice and in between there must be a period at higher temperatures. After the effect of cold in the first winter the root system can develop. However, epicotyl dormancy is broken only in the second winter. 

Examples of Epicotyl and Hypocotyl Dormancy. It is particularly well-known of our Rosaceae that their seeds require vernalization. In the case of the apple it has been found (Fig. 203) that if the seed coat is removed germination still does not take place. If, however, the thin endosperm pellicle is also removed, the embryos germinate. (We have already mentioned that the endosperm of the Rosaceae is impermeable to gases and that, for example, hydrogen cyanide formed after water uptake cannot escape.) The seedlings of other Rosaceae as well as the apple show. 

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