178 OUTLINES OF PLANT LIFE. 



Stimulated by the moisture, curve toward the damp sawdust. 

 Upon entering it the stimulus ceases, and they start again to 

 grow downward, being positively geotropic. Again the 

 stimulus of the moist surface overcomes that of gravity, and 

 they turn back to it, often threading themselves in and but 

 of the wire gauze. Since only one-sided action of a stimulus 



J 



Fig. T32. — Apparatus for demonstrating hydrotropism, a, a, a zinc dislc, witli hooks 

 to which is attached a cylinder or trough of wire netting filled with damp sawdust In 

 this are planted peas, g^ whose roots, A,/, k, nt, first descend into the air but soon turn 

 toward the damp sawdust again, w has threaded itself in and out of the netting. — 

 After Sachs. 



determines direction of movement, if the air be saturated they 

 continue to react to the stimulus of gravity alone. 



251. (e) Movements due to contact. — Contact, either 

 gentle or forcible, and friction act as stimuli to modify the 

 growth of many plant parts. Only rarely is the main axis of 

 a plant sensitive to mechanical stimuli, except, perhaps, to 

 long continued contact (or pressure) in the case of some 

 twining plants. But in many plants tendrils and leaf-stalks 

 are irritable to contact, even to a degree far surpassing that 

 of our nerves of touch. 



If the tip of a tendril (^225), while still capable of growth, 



