IRRITABILITY. 375 



movements of growing organs, and the movements of mature 

 motile organs. It must not, however, be supposed that this 

 classification is anything more than a matter of convenience. 

 There is no fundamental difference between the movements of 

 growing and of mature organs; on the contrary, they are 

 essentially similar. But there is this distinction, that whereas 

 the position assumed by a growing organ in consequence of a 

 movement may be rendered permanent and irreversible by 

 growth, the position assumed by a mature motile organ in 

 consequence of a movement is never thus rendered permanent 

 but may be changed and reassumed an indefinite number 

 of times. 



I. The Irritability of Growing Organs. 



We may regard growth as a slow movement spontaneously 

 performed by the growing organ. The effect of the action of 

 an external agent upon a growing organ is to change either 

 the rate or the direction of this movement. Confining our- 

 selves for the present to the consideration of changes in the 

 rate of growth, we will, before discussing the influence of ex- 

 ternal conditions in inducing them, enumerate the variations 

 which spontaneously occur. In the last lecture we found that 

 the rate of growth of organs presents spontaneous irregular 

 variations (stossweise Aenderungen, p. 358), as well as the 

 spontaneous regular variations which constitute the grand 

 period. These spontaneous variations in rapidity may be 

 ascribed to variations in the evolution of energy upon which 

 growth depends (p. 292), or to variations in the conditions 

 upon which, as we have seen (pp. 40, 335), the turgidity which 

 is essential to the growth of cells depends ; for instance, to 

 variations in the osmotic properties of the cell-sap, in the 

 physical properties of the primordial utricle, or, finally, in 

 those of the cell-wall. We found, further, that the rate of 

 growth is usually not uniform in all parts of the transverse 

 growing zones, so that the growth in length of an organ 

 rarely, if ever, takes place in a straight line, but that its apex 

 nutates. This nutation we found to be due to spontaneous 



