Regulation of Germination and Evolution

From the example of winter and summer annuals in the Colorado desert we have already learned that individual species show different temperature optima of germination. The examples could be multiplied. Thus, it is known that useful plants from cold climatic zones germinate better at lower temperatures and those from warm regions better at higher temperatures. Such data are indicative of an adaptation to existing circumstances. 

This leads to the question as to what the gauge is, and in answer to which one can only speculate. It may be that the content of endogenous inhibitors falls when low temperatures are allowed to act for a longer period of time. In other cases, such as the ash, an increase in the content of substances that promote germination, such as the gibberellins, and that are induced by lower temperatures seems to be more important. 

We are better informed about the nature of the gauge as far as precipitation, i.e. the germination factor water, is concerned. Precipitation in particular is the germination factor in arid and semiarid regions. That is why Went, Evenari and other researchers carried out experiments on plants from such regions in order to investigate the chemical rain gauge. 

Many annual desert plants germinate only if at least 125 mm but preferably 250-500 mm precipitation has fallen. This is practical in that the soil is then sodden enough to allow for the further development of the plant. Now the upper soil layer in which the seeds lie is just as sodden after a light precipitation of 125 mm as after a heavier one. How do the seeds measure the amount of precipitation under these circumstances  

In many cases the gauge is the inhibitors of germination, which are present in the embryo and the surrounding layers. These inhibitors of germination are washed out by rainfall and for this a certain minimal amount of precipitation is necessary. This washing out effect can easily be verified in laboratory experiments. If seeds are put on sand and sprinkled from above with at least 125 mm water they germinate. If, however, the 

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