GERMINATION. 337 



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 es- 

 sential to subsequent vegetation, is unnecessary, if not 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 

 will 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 

 the parts of the seed ; the embryo swells, and bursts its envelopes ; 

 the radicle is protruded, and, taking a downward direction, fixes it- 

 self in the soil ; while the other extremity elongates in the opposite 

 direction, bringing the cotyledons (except when these remain un- 

 der ground, as in the Pea, the Horsechestnut, Wheat, &c.) and the 

 plumule, or growing apex of the young stem, to the surface, when 

 the primordial leaves expand in the air. As soon as the root and 

 leaves are developed, each in their appropriate medium, the pro- 

 cess of germination is finished ; and the plant, deriving through 

 them its nourishment, continues to grow in the manner already 

 described (113). 



643. The nourishment which the embryo requires during germi- 

 nation is furnished by the starch, &c., of the albumen (627), when 

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

 accumulated* in its own tissue (630). 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 compounds, or neutral azotized products- 

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

 albumen or in the embryo itself (356), and which here, as else- 

 where, take the initiative in all the transformations of vegetable- 

 matter (27). Here, just as in growth from a bulb or tuber, the 

 changes essentially consist in the transformation of the starch r 

 first into dextrine, or gum, and thence into sugar (350), a part of 

 which is destroyed by resolution, first into acetic acid, and finally 

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