GERMINATION. 771 



as under such circumstances, while the light is excluded, air, 

 moisture, and warmth, have free access. Seeds thus placed 

 absorb moisture, soften, and swell ; certain chemical changes 

 go on at the same time in the substance of the albumen, or, 

 when that is absent, in the cells of the cotyledonarv portion, by 

 which a proper supply of nourishment is provided for the embryo. 

 These chemical changes chiefly consist in the conversion of starch 

 and other analogous substances which are insoluble and there- 

 fore not in a suitable state for absorption, into soluble matters 

 such as dextrine and grape sugar. The immediate cause of this 

 transformation of starch is due to a nitrogenous substance called 

 Diastase, which is developed during germination, from an al- 

 teration of a portion of the azotized contents of the seed. 

 During these changes, as already noticed, heat is evolved, and 

 carbonic acid gas driven off" fi*om the combination of the super- 

 fluous carbon of the seed with the oxygen of the air. The 

 nutriment being thus available for use, it is absorbed dissolved 

 in water by the embryo, which is thus nourished, increases in 

 size, and ultimately bursts through the integuments of the seed. 

 Its lower extremity or radicle {fig. 160, r), or one or more 

 branches from it {fig. 742, r), is commonly protruded first from 

 its proximity to the micropyle, which is the weakest point in 

 the integuments ; and by taking a direction downwards becomes 

 fixed in the soil. The opposite extremity soon elongates up- 

 wards {fig. 161, <)» and is terminated above by the plumule or 

 gemmule, which is the first terminal bud or growing apex of 

 the stem; and at the same time the cotyledonary portion is 

 either left under ground or is carried upward to the surface. 

 The embryo during this development continues to be nourished 

 from the matters contained in the albumen or cotyledonary 

 portion, and ultimately by continuing to absorb nutriment it is 

 enabled to develop its first leaves {primordial) {fig. 161, d), and 

 root {fig. 161, r), and then the young plant being placed in a 

 position to acquire the necessary nourishment by itself for its 

 further support and growth from the media by which it is sur- 

 rounded, is rendered independent of the seed, the cotyledonary 

 portion accordingly perishes, and the act of germination is 

 complete. 



Direction of Plumule and Radicle. — The cause which leads to 

 the development of the axis of the embryo in two opposite 

 directions has not yet been satisfactorily demonstrated, although 

 much has been written on the subject. By some it has been 

 referred to the action of darkness and moisture on the root, and 

 that of light and dryness on the stem. By others it has been 

 attributed to gravitation and the state of the tissues; others again, 

 regard endosmotic action as the cause. All these explanations 

 are unsatisfactory, and need not be further alluded to in an 

 elementary manual. It seems certain, however, that the chief 

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