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tinct transmission of impulse from the cells receiving the stimulation to 

 the cells a short distance away where the movement is consummated. 

 Thus, in the tip of the primary root Darwin found that only the cells at 

 the very tip were sensitive. If so small a piece as the twentieth of an inch 

 be removed from the end of the root by cutting or burning, all power of 

 movement is lost. This remarkable localization has been denied by Sachs 

 and Detlefsen, Avho characterize Darwin's claim as sensational, but the fact 

 has quite recently been fully verified by Wiesner, who finds that if 'the 

 root is weakly sensitive, the seat of irritability coincides with the zone of 

 most rapid growth, but if highly sensitive, it will be at a distance. 



To sum up the characteristics of the gravity sense: It is localized in or 

 near the ends of growing roots, stems and other organs of the plant; it is 

 developed in varying strength in different organs ; it sets up movement of 

 the organ in response to stimulation ; the direction of movement will de- 

 pend upon the specific kind of sensibility acquired by that organ; the 

 direction of the movement will always bear some definite relation to the 

 A'ertical without regard to the position of the plant. 



But, what other senses have plants ? Next to a proper position, most 

 plants need a suitable exposure to light. I shall not attempt to show the 

 numerous and wonderful ways in which plants respond to light. Every- 

 one knows how plants lighted from one side, as when placed before a 

 window, bend toward the light. This is a true sensitiveness, for it results 

 in bringing about definite movement. It is not, however, at all like see- 

 ing, for it will be noticed that it is not the amount of light, but the direc- 

 tion of light to which the organs respond. The stems place themselves 

 parallel to the direction of the incident rays — that is, point toward the 

 window, while the leaves place themselves at right angles to the direction 

 of the light — that is, with their upper surfaces to the window. Leaves 

 and stems, therefore, show a sensitiveness characteristic of each. Some 

 stems, however, like those of the Virginia creeper, turn away from the 

 light, enabling them to cling to dark walls. Roots, which are generally 

 buried in the soil, rarely exhibit sensitiveness to light, and when they do, 

 it is usually to turn from it. If light comes to the organ from two direc- 

 tions, it will bend toward the source of the stronger light, and diflferences 

 which will affect the plant are far more minute than can be detected by 

 the eye. 



As in the case of roots, certain stems place themselves, not parallel with 

 the direction of the light, but at some particular angle to it, in accordance 



