No. 502] CHEMICAL MECHANICS IN LIVING PLANT 645 



remains stationary, half-developed. If a Tropaeolum in a 

 pot be watered with dilute salt-solution, its stomata soon 

 close permanently, and no C0 2 can diffuse in to supply 

 the carbon for further growth of the plant. In such a 

 condition the plant may remain for weeks looking quite 

 healthy, but its growth may be quite in abeyance. 



In agricultural experience, in manuring the soil with 

 nitrogen and the essential secondary elements, the same 

 phenomenon is observed when there is a shortage of any 

 single element. If a continuous though inadequate sup- 

 ply of some one element is available then the crop 

 development is limited to the amount of growth cor- 

 responding to this supply. Agriculturalists have for- 

 mulated the "law of the minimum," which states 

 that the crop developed is limited by the element 

 which is minimal, i. e., most in deficit. Development 

 arrested by "nitrogen-hunger" is perhaps the commonest 

 form of this. All this is of course in accordance with ex- 

 pectation on physical-chemical principles. The quantity 

 of anabolic reaction taking place should be proportional 

 to the amount of actively reacting substances present, and 

 if any one essential substance is quite absent the whole 

 reaction must cease. It therefore seems clouding a sim- 

 ple issue and misleading to say of a plant which, from 

 the arrested development of nitrogen-hunger, starts 

 growth again when newly supplied with nitrogen, that 

 this new growth is a response to a "nitrogen stimulus." 

 It would appear rather to be only the removal of a limit- 

 ing condition. 



Let us now move on a stage. Suppose a growing plant 

 be liberally supplied with all the thirteen elements that it 

 requires, what, then, will limit its rate of growth? Fairy 

 bean-stalks that grow to the heavens in a night elude the 

 modern investigator, though some hope soon to bring back 

 that golden age with overhead electric wires and under- 

 ground bacterial inoculations. If everything is supplied, 

 the metabolism should now go on at its highest level and 

 quantities of carbon, nitrogen, hydrogen and oxygen sup- 

 plied as COo, nitrates and water will interact so that these 



