18 PLANT GROWTH 



build more protoplasm and to make more cells, thus increas- 

 ing the size of the embryo. The rate at which these numer- 

 ous activities in a germinating seed proceed in a coordinated 

 manner depends in several ways on the conditions of the en- 

 vironment with regard to temperature, moisture, and oxygen, 

 any one of which may limit the rate of development. 



Since the stored food is being used for respiration and 

 growth by the young embryo, the seedling will lose in dry 

 weight until it is able to carry on photosynthesis in the new 

 leaves to make food at least as fast as it is being used. A 

 young seedling several times the size of the seed may have a 

 dry weight of less than 60 per cent of the original seed. This 

 stage in the life of the plant is a critical one because it has 

 not developed protective tissues against insects or loss of 

 moisture and has a small reserve of food. Wheat has used 

 more than 80 per cent of its stored food during the first nine 

 days of germination, after which it is largely dependent on 

 food from photosynthesis. If seeds are planted too deeply 

 or if some other condition is unfavorable for rapid germina- 

 tion, they may exhaust the stored food before they are able 

 to synthesize it fast enough for vigorous growth. 



The fact that seedlings will grow slowly or may die be- 

 cause of lack of food may be employed in a number of ways 

 to control weeds. They are easily pulled at the seedling 

 stage because of the small root system, or they can be covered 

 and soon starve to death. Weed seedlings will be less likely 

 to establish themselves in a lawn where the grass is cut two 

 inches long than in one where it is cut less than an inch long, 

 because the additional shade retards photosynthesis, further 

 weakening the young weeds when they are deficient in food. 



If two sets of seeds are planted in soil and given compa- 

 rable conditions except that one set is kept in the dark and 

 the other in the light, the seedlings in the dark will "grow" at 

 an astonishing rate. They appear to be growing faster than 

 those in the light, but if they are examined more closely it 



