OTHER TROP1SMS. 133 



tective movements of leaves or stems. Others are sensitive- 

 ness to injurious contacts (TRAUMATROPISM), to the direction 

 of water currents (R.HEOTROPISM), to electrical currents, to 

 some extent, (ELECTROTROPISM, GALVAXOTROPISM). Sensi- 

 tiveness to mechanical strains, resulting in growth of strength- 

 ening tissues, is widespread and ecologically immensely 

 important. There are cases, too, in which the completion of 

 one growth phase gives the stimulus to start the development 

 of the next, as in the movement of some ripening fruits; this 

 principle is important (perhaps extremely important) in onto- 

 genetic development. And there are other manifestations of 

 lesser importance.* 



99. Prepare a classified table of the principal external in- 

 fluences which can be brought to bear as stimuli upon the 

 protoplasm of plants, adding the probable place and method 

 of reception of the stimulus, whether or not a response is known 

 to take place, the mechanical basis of the response, its name, 

 and its ecological significance. 



100. Prepare a synoptical essay of not over 400 words 

 upon Irritability. 



CORRELATED TOPICS. 



Analysis of the factors involved in irritable responses. 

 Significance of positive, negative, and lateral responses to the 



same stimulus. 



Mechanical i's. other explanations of irritable responses. 

 Limits of irritability in relationship to heredity; cooperation of 



both in the development of individual form. Sachs' 



theory of development. 



* This subject of Irritability, though vastly important, is not treated here more 

 fully for three reasons : (i) enough has been clone to demonstrate its fundamental 

 character and its principal manifestations; (2) time will not permit its greater ex- 

 tension in a one-year course without forcing out other topics; and (3) the further investi- 

 gation of the subject belongs rather to ecology than to pure physiology. There is, 

 however, a great deal of simple valuable and practicable experimentation upon the 

 above and related topics, to which Darwin and Acton and Detmer-Moor form very 

 good guides. Such experimentation may mostly be carried on out of doors in the 

 summer, and it could very well be made the theme for holiday summer work. 



