Power to Adjust Parts to Surroundings 245 



of irritability. Indeed, in the very highly complicated and un- 

 stable organization of the plant, it must often happen that the 

 motor or growth mechanisms are set off, quite accidentally, 

 by various wholly unrelated stimuli. Such is undoubtedly the 

 nature of many of the "mechanical responses," which by some 

 recent writers have been made the basis of all plant activities, 

 development and evolution, quite regardless of the innumerable 

 other elements and conditions entering into the constitution of 

 organisms. 



A good many additional cases of thigmotropic irritability are 

 known. Thus, the leaves of some Insectivorous Plants close upon 



FIG. 87. Corn seedlings, showing the uniformity of position assumed by the growing roots 

 and stems, respectively, from very diversely placed seeds. 



flies that alight upon them, quickly in the Venus Fly-trap, and 

 slowly in Sundew. Some stamens when touched by insects, move 

 up in such a way as to dust those visitors thoroughly with pollen, 

 thus aiding in the utilization of insects for cross-pollination of 

 flowers, of which the importance will later become apparent to 

 the reader. In these and some analogous cases, the advantage, 

 mechanism, and method of stimulation are all more or less well 

 understood. 



Geotropism. When seeds fall to earth, or are placed in the 

 ground by a gardener, they come to rest in the most diverse 

 positions, with their embryonic roots and stems pointing at any 

 and all angles. Nevertheless, as they germinate, the young 

 roots, with a singular unanimity, turn downwards and the stems 

 upwards. The same thing can be shown very clearly by ex- 



