338 THE SEED. 



into -carbonic acid and water, with the abstraction of oxygen from 

 the air, and the evolution of heat (372), while the remainder is 

 rendered directly subservient to the growth of the plantlet. The 

 reason why light, so essential to subsequent growth, impedes or 

 prevents incipient germination, becomes evident when we remem- 

 ber that it incites the decomposition of carbonic acid, and the fixa- 

 tion of carbon by the plant (344-350) ; while germination is ne- 

 cessarily attended by an opposite transformation, namely, the de- 

 struction of a portion of organized matter, with the evolution of 

 carbonic acid. 



644. In most Dicotyledonous plants, the cotyledons rise out of 



the ground, and perform more or less perfectly the 

 office of leaves, until those of the plumule expand 

 (Fig. 100- 107) : but when the cotyledons are very 

 thick and fleshy, as in the Horsechestnut, the Pea, 

 the Oak, &c., they serve merely as reservoirs of 

 nourishment, and remain under ground, that is, are 

 hypogaous in germination, the first leaves which ap- 

 pear being those of the plumule. This is also the 

 case in all Monocotyledonous plants ; in which the 

 cotyledon remains within the integuments of the 

 seed, while the radicle and plumule together pass out at or near 

 the micropyle, as shown in the germinating seed of Scirpus 

 (Fig. 466). 



645. Seeds may casually germinate while attached to the parent 

 plant, especially such as are surrounded with pulp, like those of 

 the Cucumber and Melon. The process is liable to commence in 

 wheat or other grain, when protracted warm and rainy weather 

 occurs at the period of ripening ; and the albumen becomes gluti- 

 nous and sweet, from the partial transformation of the starch into 

 gum and sugar. In the Mangrove, which forms dense thickets 

 along tropical coasts, germination commonly takes place in the 

 pericarp while the fruit remains on the tree ; and the radicle, pier- 

 cing the integuments which inclose it, elongates in the air, until it 

 reaches and fixes itself in the soft maritime mud, where such 

 trees usually grow (131) ; such a plant being, as it were, vivip- 

 arous. This very naturally takes place, also, in the seeds of hy- 



FIG, 466. The germinating seed of Scirpus, a Monocotyledonous plant: a, the cotyledon, 

 remaining within the albumen, b, inclosed in the pericarp, c ; from which the plumule (d) 

 elongates. 



