SPERMATOPHYTES: ANGIOSPEKMS 373 



ture^ necessary for the chemical changes about to be 

 made. 



The first conspicuous change noted in the seed after the 

 absorption of water is the softening of the contents, the 

 solid and insoluble starch, if that be the form of the food 

 storage, being converted by a process of digestion into 

 soluble sugar, ready for transfer. The digestive substance 

 is known as enzyme^ and the most abundant enzyme in 

 seeds is diastase^ which has the power of transforming 

 starch into a sugar. Accompanying these changes there is 

 to be noted a marked evolution of heat, so that if a large 

 mass of seeds is set to germinating, as in the process 

 known as malting, the amount of heat generated may be 

 very great. 



The first part of the embryo to protrude from the seed 

 is the tij) of the hypocotyl, thrust out by the rapid elonga- 

 tion of the upper part of the hypocotyl (Fig. 143, B). This 

 protruding and rapidly elongating tip, which is to develop 

 the root, now rapidly elongates and is very sensitive to the 

 influence of gravity, responding by developing any curva- 

 ture necessary to reach the soil. Penetrating the soil, and 

 beginning to put out lateral branches, it secures the grip 

 necessary for the extrication of other regions of the em- 

 bryo. 



After some anchorage has thus been obtained, the upper 

 part of the hypocotyl again begins a period of rapid elonga- 

 tion, which results in the development of a curvature known 

 as the " hypocotyl arch " (Figs. 143, (7, and 143, a). In 

 the case of the germinating bean this arch is the first struc- 

 ture to appear above ground, and its pull upon the seed 

 is very apt to bring it to the surface. 



Finally, the arch, in its effort to straighten, pulls the 

 cotyledons out of the seed-coats and with tliem tlie stem 

 tip, the axis of the plant straightens up (Fig. 143, a), the 

 seed-leaves and sometimes other leaves expand, and ger- 

 mination is over ; for with roots in the soil, and green 



