IRRITABILITY 207 



stem immediately below. In ordinary ortJtiotropic stems geo- 

 tropic sensitiveness and response are possible only in elon- 

 gating parts, and since the nodes and internodes soon cease 

 to grow in length, geotropic phenomena soon cease. In 

 such stems the cortical parenchyma, uniform in structure 

 and usually containing chlorophyll, is the sensitive tissue, 

 all the other tissues cooperating in the response to the 

 stimulus. In grass haulms, on the other hand, there being 

 a persistent meristem at the base of each internode and 

 in this no visible differences among the cells, we may as- 

 sume that the whole meristem is sensitive as well as re- 

 sponsive. When a grass haulm is laid prostrate by wind 

 or rain, the meristem cells on the lower side of each inter- 

 node begin again to divide, growth is resumed, and by this 

 means the parts nearer the tip are gradually carried again 

 into the vertical position. 



Prostrate and underground stems and branches, and es- 

 pecially leaves, though sensitive to gravity and assuming 

 very definite positions when acted upon by gravity alone, are 

 so much more strongly affected by light that their position, 

 as well as their form, size, and structure, must be ascribed 

 mainly to its influence rather than to gravity (see pp. 214- 

 215). 



After the growing ( motor ) zone has executed the response 

 to the stimulus received by the organ, the whole organ 

 returns, like the sense organs and muscles of an animal, to 

 that state of delicate balance and readiness in which the 

 first stimulus found it. If such a return is made impossible 

 by continued stimulation, the response will be continuous 

 until fatigue, insensibility, and impotence are produced, or 

 until the response carries the organ into such a position 

 that other influences will modify or prevent any further re- 

 sponse. This has been shown by Frank,* Darwin, f and 

 Copeland, % who found that, on confining the sensitive part. 



* Frank, A. B. Beitrage zur Pflanzenphysiologie : I. Uber die durch die 

 Schwerkraft verursaciiten Bewegungen von Pflanzentlieilen. Leipzig, 1868. 



t Darwin, P. On geotropism and the localization of the sensitive re- 

 gion. Annals of Bot., XIII., 1899. 



I Copeland, E. B. Studies on the geotropism of stems. Bot. Gazette, 

 XXIX., 1900. 



