GER'^INATION. 329 



648. The conditions requisite to germination are exposure to 

 moisture and to a certain amount of heat, varying from 50° to 80° 

 (Fahrenheit) for the plants of temperate climates, to which must be 

 added a free communication with the air. Direct light, so essential 

 to subsequent vegetation, is unnecessary, and generally unfavorable, 

 to germination. The degree of heat required to excite the latent 

 vitality of the embryo is nearly uniform in the same species, but 

 widely different in different plants ; since the common Chickweed 

 Avill germinate at a temperature not far above the freezing-point of 

 water, while the seeds of many tropical plants require a heat of 

 90° to 110° (Fahrenheit) to call them into action, and are often 

 exposed to a considerably higher temperature. Seeds are in the 

 most favorable condition for germination in spring or summer, when 

 loosely covered with soil, which excludes the light while it freely 

 admits the air, moistened by showers, and warmed by the rays of 

 the sun. The water which is slowly absorbed softens all parts of 

 the seed ; the embryo swells, and bursts its envelopes, or the elon- 

 gating radicle is protruded from them, and all the parts grow or 

 unfold in the manner already described, each organ in its proper 

 medium, the root being developed in the soil, and the stem and 

 leaves in the air. 



649. The nourishment which the embiyo requires during germi- 

 nation is furnished by the starch, &c. of the albumen (632), when 

 this substance is present in the seed ; or by starchy or other nutri- 

 tive matter accumulated in its own tissue (636, 123). But as starch 

 is insoluble in cold water, certain chemical changes are necessary to 

 bring it into a fluid state, so that it may nourish the embryo. These 

 changes are incited by the proteine or neutral azotized products 

 (354), which are largely accumulated in the seed, either in the 

 albumen or in the embryo itself, and which take the initiative in all 

 the transformations of vegetable matter (27). In the germinating 

 seed, just as in growth from a bulb or tuber, the changes essentially 

 consist in the transformation of the starch, first into dextrine, or 

 gum, and thence into sugar, a part of which is destroyed by resolu- 

 tion, first into acetic acid, and finally into carbonic acid and water, 

 with the abstraction of oxygen from the air, and the evolution of 

 heat (349, 370-373), while the remainder is rendered directly sub- 

 servient to the growth of the plantlet. The reason Avhy light, so 

 essential to subsequent growth, impedes or prevents incipient ger- 

 mination, becomes evident when we remember that it incites the 



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