246 



MOVEMENT IN PLANTS. 



(a) Place a pot plant in a box with the open side facing the light ; 

 the leaves turn towards the light, while the tendrils turn away 

 from the light towards the back of the box. 



(b) Turn the plant round through 180 ; the leaves and tendrils 

 again curve as before the tendrils show marked negative helio- 

 tropism. 



(c) Set in the pot a flat strip of wood, close to the plant, and note 

 that the tendrils spread out on coming in contact with the wood, 

 the tips swelling to form sticky discs which adhere to the wood. 

 For the first day or two the tendrils remain thin and weak, but 

 later they become thicker and stronger, and some force is needed to 

 tear them from the support. Moreover, they contract spirally after 

 becoming attached, though before contact they do not revolve in 

 the manner typical of tendrils. 



34O. Tendrils with Hooks. Get pot plants of Cobaea scan- 

 dens, or grow plants from seed. The tendrils replace the upper 

 leaflets of the compound leaf, each tendril being branched and 

 representing not only the midrib but also the veins of a leaflet. At 

 the tips there are hooks, which enable the tendril to catch on to 

 supports. 



Note that the tendrils show revolving movements before becoming 

 attached ; they are very sensitive to friction, bending over towards 

 the rubbed side, and straightening themselves again in about half 

 an hour ; a tendril will coil around a thin support in about ten 

 minutes, though unable to coil round a thick support. 



VII. EXPERIMENTS WITH MIMOSA. 



341. Specimens of the Sensitive Plant (Mimosa pu- 

 dica) may be raised from seed, even with a cool greenhouse. 

 Note the alternately arranged compound (bipinnate) leaves, 

 each leaf consisting of a main stalk, from the top of which 

 diverge four secondary stalks, each bearing numerous leaf- 

 lets in pairs. The pulvini are at the base of (1) the main 

 or primary stalk, (2) each of the secondary stalks, (3) each 

 of the leaflets. 



At the large basal primary pulvinus the movements are 

 in a vertical plane, raising or lowering the whole leaf ; the 

 movements of the four secondary pulvini cause the approxi- 

 mation or separation of the four secondary stalks ; while 



