IRRITABILITY 



35 



Reproduction, or the formation of new organisms similar to 

 the parents, is possible only for protoplasm, not for any other 

 known substance. 



46. Irritabil- 

 ity. Another 

 characteristic of 

 protoplasm is its 

 irritability. By 

 this is meant its 

 power of re- 

 sponding in 

 some way to an 

 application of 

 energy which 

 serves as a stim- 

 ulus. A famous 

 plant physiolo- 

 gist 1 has illus- 

 trated the matter 

 very simply 

 thus : A wound- 



up alarm clock, 

 which is not 



FIG. 27. Protoplasts in ovule and fruit of snowberry 



(Symphoricarpus racemows) 

 going, is given a 

 shake (stimulus), 

 which starts the 

 clock, and after 

 an interval of 

 time (latent 

 period) rings the alarm (result). The sensitiveness of the clock 

 to any jar which sets it going corresponds to the irritability of 

 living protoplasm. This extremely delicate responsiveness may 

 be manifested in a simple cell or in an organ or entire plant 

 composed <jf multitudes of cells. 



1 Professor W. Pfeffer, of Leipzig, Germany. 



A, cells from ovule (x 340) ; B, cells from an ovule further 

 developed (x 340) ; C, D, cells from pulp of fruit (x 110) ; 

 n, nucleus; p, protoplasm; s, cell sap. After Prantl 



In the young and rapidly growing cells A and B the ce}l 

 sap is not present, or present only in small quantities, 

 while in the older cells C and D it occupies a large por- 

 tion of the interior of the cell 



