CHAPTER IV. — SENSITIVENESS OF PLANTS. 255 



Fly-trap, by the stamens of the Barberry, etc. Phenomena like 

 these are by no means exceptional, though in many cases much 

 less conspicuous. In fact, all the spontaneous movements of 

 plants that have been described are evidences of irritability. 

 They take place, that is, in response to a stimulus of some sort 

 communicated to the living protoplasm. The irritant or stimu- 

 lant influence may be gravitation, light, heat, chemical agents, 

 electricity, or mechanical shock, pressure or contact. 



If we experiment upon a mass of naked, living protoplasm, 

 such, for example, as the plasmodium of one of the Myxomycetes, 

 we find that a shock causes it to contract, whether the shock be 

 that produced by a mechanical blow, or that caused by a current 

 of electricity. Also, if we strike a young and growing shoot a 

 smart blow, it will respond to the stimulus by slowly bending, 

 the character of the movement it undergoes depending on the 

 force and direction of the blow. Facts like these, and numerous 

 similar ones, justify the conclusion already stated, that irrita- 

 bility is a property common to all living vegetable protoplasm. 



Among the more interesting phenomena of this kind are the 

 sensitiveness of tendrils and other climbing organs. Take, for 

 example, tne tendril of the Passion-flower. When young, it is 

 straightened out and somewhat hooked at the apex, and is car- 

 ried around in a circle by circumnutating movements. If, in the 

 course of these, it fails to be brought into contact with an object 

 suitable for it to cling to, it soon coils up into a close spiral, loses 

 its sensitiveness, and finally withers away. But if the hook at its 

 extremity comes into contact with the stem or branch of another 

 plant or other suitable object, it is likely to catch upon it, the 

 irritation of the contact causes it to bend around it and clasp it 

 firmly, ultimately, if the shape of the object permits, forming 

 two or three coils about it. The rest of the tendril then forms a 

 double spiral, a part of which winds in one direction, and the 

 rest in the opposite direction, and at the same time its tissues 

 acquire great firmness and elasticity. Ttoe spiral coils thus 

 formed serve the double use of drawing the plant closer to its 

 support, and of acting as a spiral spring to prevent it from being 

 torn away by a sudden strain, such as that produced by a gust 

 of wind. 



The tendrils of some species of Ampelopsis, as we have seen, 



