THE MOVEMENTS OF PLANTS. 203 



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

 determines direction of movement, if the air be saturated 

 they continue to react to the stimulus of gravity alone. 



293. (c) 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 



<t 



FIG. 200. Apparatus for demonstrating hydrptropism. a, a, a zinc disk, with hooks 

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

 this are planted peas, g, whose roots, /z, /, k, in, first descend into the air but soon turn 

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

 After Sachs. 



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

 plants. But in many plants lateral axes in the form of ten- 

 drils (H^l 115, 158) and leaf- stalks (^| 157) are irritable to 

 contact, even to a degree far surpassing that of our nerves of 

 touch. 



If the tip of a tendril (^j 266), while still capable of growth, 

 come in contact with a solid body, it will quickly become 

 concave on the side touched, and thus will wrap about the 

 object, if it be of suitable size. This curvature is due first to 

 the shortening of the cells upon the concave side and later 

 to unequal growth on opposite sides. Finally this effect 



