176 ELEMENTARY STUDIES IN BOTANY 



to animals as well as to plants. Not only does the germinat- 

 ing seed give off carbon dioxide, but also heat. This becomes 

 very evident in the process of malting, in which a large mass 

 of barley is put in germinating conditions in a confined space, 

 and the total heat developed by the mass of germinating 

 seeds is so great that the water may become scalding hot. 



107. Digestion. Before the embryo can grow, the food 

 stored in the endosperm or in the embryo itself must be 

 changed from its storage form to its transfer form ; that is, 

 from an insoluble form to a soluble form, so that it may move 

 freely in solution. The process resulting in this change is 

 called digestion (see 30, p. 38). A very common storage 

 form of food in seeds is starch, and by digestion the insoluble 

 starch is converted into a soluble sugar. The active agents 

 in this process are enzymes, substances produced by the pro- 

 toplasts, and the particular enzyme that converts the starch 

 of a seed into sugar is diastase. As a result of digestion, the 

 seed, which had become swollen by the entrance of water, is 

 observed to " soften." It is at this stage that the germi- 

 nation of the seed is checked in the process of malting, which 

 thus secures the carbohydrates of the seed in the form of a 

 solution of sugar. 



108. Assimilation. After digestion, the food in solution 

 moves toward the active cells (protoplasts) of the embryo, 

 in proportion to their activity. Growth is not uniform in 

 amount throughout the embryo, so that some regions receive 

 more food supply than others. The most actively growing 

 region of the embryo at first is the hypocotyl (see 75, p. 126), 

 and in its cells the protoplasts are most active. The pro- 

 toplasts use the food received by transforming it, step by 

 step, into living protoplasm, and it is this process that is 

 called assimilation (see 30, p. 38) . The protoplasmic body 

 of the protoplast is used up in providing the materials for 

 growth, and therefore it must be built up continually by 

 assimilation. 



