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 



going, is given a ^ cells from ovu]e ( x 340 ) . ^ cel i s f r0 m an ovule further 

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

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



n-' 



FIG. 27. Protoplasts in ovule and fruit of snowberry 

 (Symphoricarpus racemosus) 



In the young and rapidly growing cells A and B the cell 

 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 



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 of multitudes of cells. 



1 Professor W. Pfeffer, of Leipzig, Germany. 



