SENSITIVENESS AND IRRITABILITY OF PLANT ORGANS. 155 



light is that of the prothallium cell.* Climbing roots of 

 Ivy also appear on the darker side of the shoot, etc. 



It is impossible to regard the above cases as isolated, but 

 they are special instances, revealing not only the general 

 irritability of protoplasm, but the minuter effects upon the 

 nucleus, which, in its turn, is thus compelled " to respond," 

 and sets up cell-division, i.e. the formation of a tissue in the 

 direction of the external influence, as mentioned above in the 

 sentence I have italicized. 



The next very important point to notice is that cell-divi- 

 sion can take place in response to, and in the direction of an 

 external mechanical stimulus, just as well as in that of light. 

 As the sensitive plant is influenced by, and visibly moves its 

 foliage under the irritation of a touch or of varying degrees 

 of light, so do I assume that the peculiar anatomical 

 structures which permit of those motions are the direct result 

 of external stimuli. Sjparmannia, it may be added, exhibits 

 three kinds of movement, viz., Sleep in the calyx and corolla, 

 mechanical irritahility in the stamens, and an elevation of the 

 peduncle. (See Heckel, I.e., p. 210.) If this position be 

 granted we have at least a working hypothesis for the 

 present theory of the origin of floral structures. 



Formation of Tissues due to Irritability. — Apart from 

 the preceding theoretical supposition, there may be fre- 

 quently witnessed an actual formation of tissues of various 

 kinds, through hypertrophy on the one hand, often coupled 

 with atrophy on the other, and entirely brought about by 

 physical or mechanical irritations. Cell-division is thus set 

 up, a result which would not have occurred had not the 

 external stimulus been applied. 



It is an important fact to notice, that in some cases the 

 abnormal growth, though immediately following the stimulus, 

 * See Jl. Roy. Micr. Soc, 1886, p. 287 ; and Bull. Soc. Bot. Fr., 21, p. 65. 



