TUOPISMS 



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Many of the movements of plants and parts of plants arc 

 stimulated by light, heat, gravity, electricity, etc. Tiie plant re- 

 sponds to these external conditions. 

 Movements due to the stimulation 

 of external force are called tropic, 

 A rooted plant is so constituted that 

 its root grows downwards and its 

 stem upwards, and if the root be 

 placed horizontally it will in time 

 bend downwards. The root is 

 positively geotropic — seeking the 

 earth — whereas the stalk is negatively 

 geotropic — avoiding the earth. These 

 movements are, of course, of value to 

 the plant inasmuch as they help to keep 

 its various organs in the right places 

 for carrying on their functions. 



Light plays a large part in the 

 movements of the various organs of 

 the plant. Like the ex-Kaiser, green 

 plants seek "a place in the sun." The 

 majority of plants are heliotropic ; thus 

 the ordinary green leaf places itself so 

 that it can receive the largest number of 

 rays of light. Roots seldom or never con- 

 tain chlorophyll and are negatively helio- 

 tropic. Certain chemical substances which 

 pass from the cells of the plant into the sur- 

 rounding media induce movements in a given 

 direction, for instance, the antherozoids of the 

 bracken-fern are said to be attracted to the 

 egg-cell by the excretion of malic acid. 



Other movements are brought about by 

 touch. The leaves of the sundew and the Pio. 39. Seedling 

 Venus' fly-trap close up and entomb anv of tlie Bean, Vina 

 insect which rests on their surface. The f'''^^[^ ?^;,^*j[" ^l^^;^ 

 stamens of the barberry and of some orchids win's Elements of 

 also move when touched, and tlien again we Botany.) 

 have the well-known sleep movements; the leaves of the 



