PROCESSES PREPARATORY TO GERMINATION 377 



certain temperature the working of the protoplasmic 

 "machine " is thrown out of gear and death results. 



Enzyme Formation and Activity. — One of the first 

 processes preparatory to germination is the production 

 of enzymes in the cells of the endosperm and embryo, 

 by the activity of which the various reserve foods are 

 converted into soluble forms. The starch is acted 

 upon by diastase, the cellulose (as in the date seed) 

 by cytase, the fats by the lipases, the proteins by 

 various proteases (see p. 45). The carbohydrates and 

 fats thus give rise to great quantities of sugars, and 

 these are largely used for respiration, i.e. for the libera- 

 tion of energy which is seen in the active growth of the 

 embryo : part' of this energy takes the form of heat. 

 Thus the main interchanges between the germinating 

 seed and its surroundings are first of all the active 

 absorption of water resulting in the swelling of the seed 

 (cf. the experiment with peas, p. 60), then the absorption 

 of free oxygen, and the evolution of large quantities 

 of carbon dioxide and heat (p. 89) . Some of the sugar 

 is used for forming new cell walls, while the soluble 

 nitrogenous substances into which the solid proteins 

 are converted by the activity of proteolytic enzymes 

 are worked up again to form the basis of the new 

 protoplasm produced in active cell division. 



Structural Changes in Germination. Epigeal and 

 Hypogeal Cotyledons. — Most of the cells of the embryo, 

 especially if it is small and relatively slightly developed 

 in the resting stage, actively divide during the first 

 stage of growth, but cell division soon becomes mainly 

 concentrated in the apical meristems. The actual 

 growth of the embryo into the seedhng takes place 

 very largely by vacuolation and consequent increase 

 in size of the cells. 



