788 PHYSIOLOGY. 



sugar. The immediate cause of this transformation of starch is 

 due to a nitrogenoiis substance called by Payen diastase, which 

 is developed, during germination, from an alteration of a portion 

 of the azotised contents of the seed. During these chemical 

 actions, heat is evolved, as in the malting of Barley, and car- 

 bonic acid gas given off from the combination of the super- 

 fluous carbon in the starch and albuminoids with the oxygen of 

 the air. The nutriment being made available for use, it is ab- 

 sorbed 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 {Jig. 14, r), or one or 

 more branches from it {fig, 745, 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 be- 

 comes fixed in the soil, whilst soon after the opposite extremity 

 elongates upwards {fig. 14, t^, and is terminated above by the 

 plumule or gemmule, which is the first terminal bud or growing 

 apex of the stem. At the same time the eotyledonary portion is 

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

 The embryo during this development continues to be nourished 

 from the matters contained either in the albumen or eotyle- 

 donary portion, and ultimately by continuing to absorb nutri- 

 ment it is enabled to develope its first leaves {primordial) {fig. 

 15, d, d), and root {fig. 15, r). The young plant is now placed 

 in a position to acquire the necessary nourishment for its further 

 support and growtli from the media by which it is surrounded, 

 and is rendered independent of the seed; the eotyledonary 

 portion accordingly perishes, and the act of germination is 

 complete. 



Direction of Plumrde 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. Ey some it lias 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 osmotic action as the cause. All these explanations are 

 unsatisfactory, and need not be further alluded to. Darkness 

 has been shown to have no influence on the direction of the root, 

 which is perhaps determined by the greater amount of moisture 

 usually met with in the soil. In Trwpa natans the radicle is 

 directed upwards towards the surface of the water in which the 

 plant grows. 



Differences between the Germmation of Dicotyledonous and 

 Monocotyledonous Seeds. — There are certain differences between 

 the germination of Monocotyledonous and Dicotyledonous em- 

 bryos, which have already been alluded to briefly (see pp. 120 

 and 121), but which require some further notice. 



