GERMINATION AND GROWTH 



51 



chanical effect of gravity, due to weight of parts, as when the 

 bough of a fruit tree is bent under the load of its crop, but 

 a certain stimulus to which the plant reacts by a spontaneous 

 adjustment of- its growing parts. In other words, geotro- 

 pism is an active, not a passive function, and the plant will 

 overcome considerable resistance in response to it. (Exp. 38). 

 54. Other factors. - - The direction of growth is influ- 

 enced by many other factors, such as light, heat, moisture, 



contact with other bodies, 

 electricity. The result of all 

 endless variety in the forms 

 organs that seems to defy 

 Heat, unless excessive, gen- 

 growth ; contact sometimes 

 causing the stem to curve 

 turbing object, and sometimes 

 the stem to curve toward the 

 by growing more rapidly on 



and perhaps by 

 these forces is an 

 and growth of 

 all law. 



erally stimulates 

 stimulates it, 

 away from the c 1 i s- 

 retards it, causing 

 object of contact 

 the opposite side, 



FIG. 70. A piece of a haulm of millet that has been laid horizontally, righting 

 itself through the combined influence of contact and negative geotropism. 



as in the stems of twining vines. Light stimulates nutrition, 

 but generally retards growth. The movements of plants 

 toward the light are effected in this way; growth being 

 checked on that side, the plant bends toward the light. 



Practical Questions 



1. Why do stems of corn, wheat, rye, etc., straighten themselves after 

 being prostrated by the wind ? (51, 54.) 



2. Do plants grow more rapidly in the daytime, or at night ? (54.) 



3. Reconcile this with the fact that green plants will die if deprived 

 of light. 



