24 PLANTS 



48. That oxygen is necessary to germination may be demon- 

 strated by experiment either by placing the seeds of aquatic 

 plants in water from which the air has previously been expelled 

 by boiling, or by placing seeds in a vessel containing an atmos- 

 phere deprived of its oxygen. 



49. Under the conditions just enumerated the embryo swells 

 through the absorption of water, the seed coats burst and the 

 caulicle grows out and down into the soil, the terminal part of 

 it going to form the primary root. It will be noted here that 

 the direction of growth, not only of the root, but also of the other 

 growing parts of the plant, is very definite and that the deter- 

 mining cause of it must be sought in some external agency. 

 By suitably conducted experiments we find that gravity acts 

 upon the primary root as a stimulus, in response to which it 

 grows downward. This response is known as positive geotrop- 

 ism. If the influence of gravity be eliminated the root will turn 

 toward the source of moisture this is positive hydrotropism. 



50. The plumule also responds to external stimuli, but in 

 a different way. It turns away from the earth, being negatively 

 geotropic, and grows toward the light positively heliotropic. 

 (The student should note that unless light actually impinges 

 on the seedling it can have no influence in determining the 

 direction of growth. Hence, if the seed is growing in the 

 dark the direction of growth must be determined by some 

 stimulus other than light. In this connection analyze care- 

 fully the results of experiments i and 2 under paragraph 30, 

 and 31, page 10.) 



The Seedling 



51. When the primary root has penetrated the soil some 

 distance, lateral branches begin to appear on all sides of it 

 at some distance above its tip. These branches are not posi- 

 tively geotropic, since they grow in an almost horizontal direc- 

 tion diageotropism with perhaps a slight tendency down- 



