IRRITABLE RESPONSE 239 



Upon this basis the student should work up this subject 

 through the literature, giving somewhat special attention to the 

 mechanism of the responses, and to acclimatization to stimuli. 

 Here also he may best consider the results following a summation 

 0] stimuli, a competition 0} stimuli, and a substitution 0] stimuli 

 acting upon the same part. And this will lead naturally to a 

 study of the essential nature 0} a stimulus, with the characteristics 

 it must present in order to act as a stimulus at all; and this in 

 turn will involve the problem of the relation between irritability 

 on the one hand, and sensation with reflex action on the other. 



(e) Chemotropism. 



Of very much importance, even though not very conspicu- 

 ous, are responses to chemical substances. For the most part 

 the experimental study is somewhat difficult, though not impracti- 

 cable, and the student should follow, experimentally as far as 

 possible, and then through the literature, the following matters : 



(a) Chemotaxis of the antherozoids of Ferns and of Bacteria 

 (compare Pfeffer's fine experimental methods, and Buller, Annals 

 of Botany, 14, 1900, 543). 



(6) Chemotropism (with aerotropism) of pollen tubes, hyphae of 

 Fungi, young roots (compare Newcombe and Rhodes in Botanical 

 Gazette, 37, 1904, 23, and Lilienfeld in Beihefte zum botanischen 

 Centralblatt, 19, 1906, 131). 



(c) Gall and other structural formations ; also fruit formation after 

 fertilization. 



(d) Regulation of the growth of petioles of water-plants, upon 

 which a striking and valuable experiment is practicable. 



Suggested Experiment. Select a typical water-plant of the floating- 

 leaf type (e.g., Limnocharis, Water Poppy) growing in a tank in a few inches 

 of water. Suspend over the tank a large glass tube, some 4 or 5 cm. inter- 

 nal diameter and a meter long, closed at the upper end and nearly filled 

 with water. Into this tube guide a young developing leaf. Beside this 

 tube place another also closed above, but filled with air; depress it to near 

 the bottom of the tank, and guide into it a young leaf. Thus the leaves are 

 supplied with three different water-levels. Compare the relative growths of 

 the petioles of the various leaves, and observe the cellular basis in the petioles. 

 Also with other leaves replace the air in both tubes by nitrogen or hydrogen, 

 and observe the result upon petioles. (Upon this illuminating experiment 

 compare Karsten in Botanische Zeitung, 49, 1888, 565.) 



