250 LECTURES TO SCIENCE TEACHERS. 



prevent as far as possible the action of other agents upon 

 the organism under experiment, and this may be done 

 either by entirely removing the agents in question, or by 

 rendering this action constant throughout the experiment. 

 In the present case the disturbing agents are light and 

 moisture. The action of the former may be prevented by 

 keeping the apparatus in the dark, and of the latter, by 

 maintaining the atmosphere within the case at its point of 

 saturation. 



The first and simplest experiment which we might 

 perform would be one in which a plant should be exposed 

 to a constantly increasing temperature, and we should find 

 that when a certain degree in temperature was reached, 

 the plant would die. In the next place, we should find 

 that, if a plant were exposed to a constantly decreasing 

 temperature, after a time death would ensue. From these 

 experiments we should learn that plant life can be main- 

 tained only within certain limits of temperature and by 

 repetitions of these experiments we should be able to 

 determine thesa limits with accuracy. Roughly speaking, 

 they are O C. (32 F.) and 50 C, (122 F.) (Sachs). 



The next step would be to investigate the relation 

 existing between each one of the phenomena of plant-life 

 and the temperature of the surrounding medium. Suppose, 

 for instance, that we are investigating the effect of tempera- 

 ture on growth. Taking the Pea as the subject of experi- 

 ment, we should find that it would not grow at all at a tem- 

 perature lower than 6 C., that the rapidity of its growth 

 would increase with every rise of temperature up to 26 C. 

 that any further rise of temperature would be attended 

 with diminished rapidity of growth, and that, at a tempera- 

 ture of about 40 C., growth would be entirely arrested 

 (Koppen). 



A series of such experiments upon any plant would 

 show that all its functions are affected by temperature in 

 the manner just described with reference to growth. We 

 should find that, for each function, there were certain 

 definite limits of temprature within which that function 

 could be performed, and that between these limits there 

 was a degree of temperature which corresponded to the 

 maximum activity of the function in question. 



The next apparatus in the catalogue (No. 3935) is but a 



