J32 PLANT PHYSIOLOGY. 



Observe the operation of the tendrils naturally clasping a 

 support. 



What is the rate of 4 ' revolution ' ' of the tendrils ? 



98. What phenomena in response to various stimuli does the 

 common Sensitive Plant show ? 



Answer by experiments (Experiment 60) devised by your- 

 selves. 



Can all of the responses to different stimuli thus shown be 

 adaptive ? 



Do the responses continue actively if the stimulus is often 

 repeated with equal intensity ? 



What is your explanation of the extreme sensitiveness to 

 contact shown by this plant ? 



Another class of contact-effects are those shown by the irritable sta- 

 mens of Barberry, Sparmannia, etc., and by the irritable stigmas of cer- 

 tain species of Mimulus, all very easy of experiment. Still another class 

 is the growth of haustoria of parasites in contact with the host. The 

 twining of some stems, e.g. Cuscuta, is in response to contact with the 

 supporting host. 



E. Other Tropisins and Manifestations of Irritability. 



In addition to the above-mentioned four leading manifesta- 

 tions of Irritability, plants show many others, in which the 

 responses to the stimuli are, as in the above cases, definite and 

 adaptive. Some of the remaining manifestations are of great 

 ecological importance. Most important of them is response to 

 chemical substances (CHEMOTROPISM) ; in addition to its 

 ordinary phases, best illustrated by roots and fungal hyphoe, 

 there are special phases, such as sensitiveness to oxygen 

 (AEROTROPISM), shown in Pollen-tubes, and, in another way, 

 by the developing petioles of water-plants ; yet another phase 

 is the formation of galls and analogous structures ; still another 

 (partially chemotropic) is the development and ripening of 

 fruits after fertilization. Important also is sensitiveness to heat 

 (THERMOTROPISM), manifest in roots and stems, and u*ith a 

 special phase in which heat and cold give the stimulus to pro- 



