﻿1903] CHEMICAL STIMULATION 85 



Immediately following the before-mentioned work by 

 Kahlenberg" and myself in the spring of 1899, ^ ^started a series 

 of water cultures in which Zn and Cu were present. The results 

 were such as to encourage a continuance of the work, and 

 showed that the acceleration of growth is most decided by solu- 

 tions not a great deal more dilute than those that are distinctly 

 toxic. In fact, the concentrations producing the two effects 

 differed by less than the individual variations of the seedlings 

 used, so that, as in our earlier work when the metals themselves 

 were placed in the water, it sometimes happened that in the same 

 culture (vessel) some plants were evidently injured, while others 

 grew exceptionally well. It is evidently reasonable, even if out 

 of accord with our usual way of looking at it, to regard the 

 acceleration of growth as itself an injury to the plant. 



In the summer of 1899 appeared Jacobi's^*^ work on the influ- 

 ence of various substances on the respiration and assimilation of 

 water plants. I was immediately impressed with the superiority 

 of respiration to growth as a true index of the plant's activity. 

 I need not enter into a discussion of the relation of respiration 

 to the plant's multifarious other manifestations of activity. It 

 makes available the energy for most of them; without it all 

 would cease. Growth is dependent on respiration, and under 

 ordinary conditions they vary together; when they do not, it is 

 respiration which reveals the plant's real activity. From the 

 minimum to the optimum temperatures for growth, its curve 

 rises with that of respiration, the more rapid combustion in the 

 plant furnishing more energy- and proper material for growth. 

 But the more active the respiration, the less the relative amount 

 of plastic material left available for growth. A point must be 

 reached, then, beyond which an increase in respiration can 

 reasonably be regarded as itself the cause of a decrease in 

 growth. This is what happens at temperatures above the opti- 

 mum for growth. How far this loss of material can go during 

 the most rapid respiration is shown by Kraus's^^ classic work, in 



^Jacobi, B., Ueber den Einfluss verschiedener Substanzen auf die Athmung und 

 Assimilation submerser Pflanzen. Flora 86 : 289-327. lS99- 



»7Kraus, G., Ueber die Bliithenwarme bei Anim italicunu Abhandl. Naturf. 

 Gesells. Halle i6:— . 1884. Also, Ueber Bluthenwarme bei Cycadeen, Palmen und 

 Araceen. Ann. Jard. Bot. Buitenzorg 13:217-275. 1896. 



'\\^-l 



