186 PLANT PHYSIOLOGY 



isms themselves. To put this in more definite terms we may 

 say that the irritability (which we may represent as x) of 

 a cell, an organ, or an organism, consists in the sum of 

 these factors, viz. 



(a) the sensitiveness of the component atoms of complex 

 molecules to other forces and affinities as well as to the 

 affinities which hold them together in these compounds. 



(b) the instability of the groups of molecules. 



(c) the instability of the protoplasmic structure which 

 consists of water and these complex molecules. 



(d) the number, variety, and speed of the chemical 

 changes taking place in this structure. 



(e) the number, variety, and speed of the chemical 

 changes taking place between its component molecules and 

 others enclosed among them or outside. 



( f) the number, kinds, and degree of dissociation, of the 

 atoms and molecules of the substances dissolved in the 

 water in the cell. 



Thus x='a + 5 + c+d+e + /*, a sum greater than is 

 attained in any known combination except the living cell. 



Let us pass on now from these general considerations as 

 a starting-point, to examine certain phases of irritability : 

 first, the relations of irritability to the amount, kind, etc., 

 of growth ; second, the direction of growth, and movement, 

 as depending upon irritability; third, the growth move- 

 ments not evidently connected with irritability. 



IRRITABILITY AND THE AMOUNT AND KIND OF GROWTH 



Every actively growing organism must have, besides an 

 adequate supply of material, an adequate amount of room, 

 and the impulse to grow ( see p. 168 ) , at least the power to 

 direct its growth according to its environment. This power 

 is dependant upon the irritability of the organism and of its 

 separate organs, their sensitiveness to forces and influences 

 wholly external. Since the material of which it is composed 

 came into existence, every organism has been subjected to 

 external influences, some momentary or unequal, some per- 

 sistent and uniform. The effects of these influences are more 

 or less enduring, like the influences themselves, but presuma- 



