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with some inherent necessity. Not as many parts of the plant, as a rule, 

 are sensitive to light as to gravitation, but the degree of development of 

 the sense is often greater. 



Some plants also show a sensitiveness to moisture, especially in their 

 roots, causing them to bend toward or away from the moist surface. Cer- 

 tain molds are remarkably sensitive in this way. Errara presented a paper 

 before the British Association, last year, in which he gave the results of 

 his experiments with Phycomyces nitens, a tall-growing mold. It proved to 

 be so sensitive that the experimenter was enabled to detect the hygroscopic 

 character of certain substances not before known to be in the least degree 

 hygroscopic. Thus it bent toward alum, and careful physical tests showed 

 that alum was truly hygroscopic to a minute degree, although the prop- 

 erty had never before been ascribed to it. 



Certain plants are also sensitive to heat. Here, again, it is the direction 

 of the radient energy, rather than the amount, to which they respond. 

 In my own laboratory, experiments have shown that young plants of corn 

 will bend toward the source of heat, which in this case was a lamp placed 

 behind a screen of blackened tin, while beans bent away from it. 



But probably the most varied and wonderful of all the plant senses is 

 the sensitiveness to contact. In the animal the somewhat similar sense 

 of touch is more diffused over the body, and takes on more variety than 

 any of the other senses, and in plants it has even greater diversity than 

 in animals. In the tendrils of certain plants, notably in the passion vine 

 {Passiflora cxrulea) "this sensitiveness is often exquisitely fine, indeed, it 

 seems more delicate than the tactile sense of animals." Unlike the other 

 plant senses, it has risen above the necessity of being confined to young, 

 growing parts, and sometimes resides in special organs, as in the cushions 

 on the leaves of the sensitive plant, by which they are able to suddenly 

 shut up tightly when touched, or in the prehensile-like tentacles of the 

 leaves of sundew, which shut over and catch a live insect and secure it for 

 digestion by the plant. 



Plants are thus seen to react sensitively to gravity, light, moisture, heat 

 and contact. Each is a special kind of sensitiveness, having its own 

 method of reaction. Two or more kinds of sensitiveness may reside in 

 the same organ, when its position will be a resultant of the several forces. 

 There are, consequently, no exclusive organs of sense, although there 

 is more or less localization in certain parts; and there are no nerves, 

 although the motor impulse may be transmitted some distance, even as 



