THE SENSES OF PLANTS. 



517 



iJie light by the internod© twisting during growth. Snowberry and 

 Privet illustrate this procedure. 



The Sense of Touch. — The sense of touch is exhibited by plants in 

 many ways. Let us take first sensitive and insectivorous plants. The 

 former type may be represented by Mimosa pudica. As soon as the 

 tip of the compound leaf is touched the leaflets fold themselves 

 together 'from the end to the base, and then the petiole droops. 



The Sundew and Venus' Fly-trap afford good illustrations of the 

 effects of touch. In the former, the tentacle-bearing glands around 

 the margin of the leaf-blade bend inwards when the glands on the 

 surface of the blade, which secrete a sticky fluid, are touched, whether 

 by an inorganic or organic and digestible substance. So slight a thing 



"s^TTT P^^^ ^ grain of hair caused inflexion. Nitrogenous sub- 

 stances, which alone are digested, are most effective. 



The leaf-blade of the Venus' Fly-trap (Dionaea) bears three bristles 

 on each half-blade. One or more being touched, the two halves in- 

 stantly close up. The reader will find abundance of other interesting 

 matters in Darwin's " Insectivorous Plants." I am only here con- 

 cerned with that of sensitive living protoplasm being the instrument 

 of movement in response to touch, as therein explained. As another 

 example w^e find it developed in climbing plants. We all know how 

 sticks are necessary for Sweet Peas ; for the leaflets, which are mostly 

 represented by tendrils, have become very sensitive to touch ; so that 

 as soon as contact takes place with a twig, by a branch of the tendril 

 touching it, this twists round the latter. Previous to contact, prepara- 

 tions are made — thus the leaflets, having been all originally in one 

 horizontal plane, have now only the midribs remaining; but to provide 

 greater facility for catching, they grow out at any angle and are hooked, 

 so that they assume the form of a complex grappling instrument. 

 Moreover, the main shoot and tendrils " circumnutate, " or keep 

 " bowing around," so as to have a better chance of catching something. 



In the Ivy, the climbing shoots have acquired the habit of grow- 

 ing towards the less illuminated side in preparation for climbing up a 

 wall. Though the formation of roots on that side v/as undoubtedly 

 at first due to contact, they are now formed on the darker side before 

 contact ; but such are not completed until contact is made, though they 

 are at once formed when the apex itself reaches the wall. 



In the case of the Virginia Creeper, certain flowering branches are 

 converted into branching tendrils, by means of which it can climb up 

 a trellis, &c. ; but it subsequently acquired the habit of climbing up a 

 rough surface; so the shoots now grow towards the darker side, and 

 every branchlet of the tendril has a hooked tip which catches the 

 roughnesses of the wall, causing it to remain until the tip has developed 

 an adhesive pad. This, however, is never made in advance or before 

 contact in this species. 



In the Japanese species, however, the pads begin to be formed 

 before the contact takes place, but are not completed until contact 

 occurs. This state also obtains in some other climbers. 



