GERMINATION OF SEEDS 17 



three months before cucumbers and zinnias. Personal ex- 

 perience and suggestions from seed catalogues are good 

 guides for the observant gardener in determining the best 

 planting conditions. 



The physiology within the seed resulting in germination 

 is regulated by three external conditions: temperature, mois- 

 ture, and oxygen. Since food is stored for early growth only 

 water and oxygen must be absorbed. The following changes 

 take place simultaneously during germination in a closely 

 connected and related way. (1) Water diffusing through 

 the seed causes it to swell and weaken the seed coat. (2) 

 The enzymes become activated and dissolved in the water 

 and diffused to the stored food. The enzymes, or the sub- 

 stances which become enzymes, appear to be in the dry seeds 

 and sometimes in localized areas; for example, the enzymes 

 of the corn seed are in the epithelial layer (Fig. 1) . The rate 

 of enzyme action increases rapidly with an increase in 

 temperature, in fact in many cases an increase of 18° F. will 

 double the action of the enzyme. With a favorable tem- 

 perature, the enzyme actions in a germinating seedling ap- 

 pear to supply soluble food materials, as indicated by their 

 abundance, more rapidly than they can be used. (3) The 

 stored food of the endosperm or cotyledons is made soluble 

 and diffusible by the enzymes: large molecules of starch, 

 insoluble in water, are broken into smaller molecules of water- 

 soluble sugar; molecules of fats are converted into fatty acids; 

 and molecules of proteins into amino acids. (4) The food 

 diffuses to the growing parts of the embryo. (5) Part of 

 the food is used in respiration or plant oxidation^ to supply 

 the embryo with energy. (6) Part of the food is used to 



1 The process of respiration illustrates the kind of oxidation taking place in plants 

 and is the reason for their need of oxygen in the germination process. This can be tested 

 by putting a quantity of soaked seeds in a thermos bottle and measuring the increased 

 temperature at the end of one or two days. The experiment is more successful if a small 

 bottle of sodium or potassium hydroxide solution is placed in the bottom of the thermos 

 bottle to absorb the carbon dioxide from the respiring seeds. 



