390 TEXTBOOK OF BOTANY 



During the germination of peas, scarlet runner beans, corn, sorghum, 

 and acorns the cotyledons remain in the soil because the hypocotyl does 

 not elongate. In peas, scarlet runner beans, and acorns the initial food of 

 the seedling comes from the cotyledons of the embryo; in corn and 

 sorghum, from both the embryo and the endosperm. 



From our previous study of physiological processes in the vegetative 

 organs of plants, v^e have much of the information needed to under- 

 stand the changes that take place during gennination and the early 

 seedling stages. The v^ater content of "dry" seeds, such as com and 

 wheat, is insufficient for growth, and the seeds remain dormant until 

 water is added. The absorption of water results in a swelling of the tissues 

 and a renewal of the many cell processes that had been active during 

 the initial growth of the embryo. Water is a medium in which enzyme 

 activities proceed, and it also combines with various insoluble foods 

 during their digestion, or hydrolysis, to soluble compounds. It is like- 

 wise the medium in which soluble foods and soluble inorganic salts move 

 or are moved about in the developing seedling. The greater part of the 

 enlargement of the seedling is the result of the osmotic absorption of 

 water and the increase in size of the vacuoles of the enlarging cells. 



Oxygen is essential during germination because of the many oxidation- 

 reduction processes that occur in the complex protoplasmic system, espe- 

 cially in the transformation of foods into more permanent cell structures. 



"Favorable" temperatures obviously refer to those temperatures in 

 which the complex colloidal system of the protoplasm and physiological 

 activity are best maintained, since upon this condition permeability and 

 the coordination of many essential physical and chemical processes de- 

 pend. But temperatures ordinarily best for the rapid growth of plants 

 are not best for the geiTnination of all kinds of seeds. 



The upward growth of the shoot and the downward growth of roots 

 of seedlings are primarily the result of the distribution and influence of 

 hormones described in Chapter XXIII. 



Changes in foods during germination. The chemical changes that occur 

 in germinating seeds have been studied by means of gross chemical 

 analyses at frequent intervals during the germination process. The 

 changes have also been followed by observation of small seeds, or thin 

 sections of seeds, through the microscope with appropriate chemical 

 tests. It is not difficult to follow the transformation of starch, for example, 

 as it is hydrolyzed to dextrin and sugar. The determination of the dif- 

 ferent sugars is somewhat more difficult, as described in Chapter XII. 



