30 STUDIES IN PHYSIOLOGY. 



acid. 1 Later, double and quadruple the quantity added, and 

 observe the effect produced upon the plant. Experiment with 

 different species of ferns and club mosses in the same manner. 

 Observe in another series of experiments the effect of sunlight 

 in modifying the influence of an excess of carbonic acid gas in 

 the atmosphere. 



The 'measure of assimilative activity is to be found either 

 in the amount of pure ox3'gen evolved in assimilation, or in the 

 amount of carbonic acid decomposed in it. 



1. Determinations depending upon the amount of oxygen 

 evolved : The gas which is given off during assimilation, espe- 

 cially b}' water plants, is never absolutel}' pure oxygen ; but 

 since it contains so small a proportion of other matters under 

 most circumstances which the student is likely to meet, the 

 amount of it evolved may be taken safety as the approximate 

 measure of assimilation. The method of measurement b}' count- 

 ing bubbles emitted by water plants in water (see 814) is always 

 practicable and eas} T of execution. The evolved gas can be 

 easily collected in an}' convenient inverted receptacle. If the gas 

 collected and measured is analyzed eudiometrically, as deleted 

 in Buusen's " Gasometiy," the determination leaves little to be 

 desired. 



2. Determinations depending upon the amount of carbonic 

 acid decomposed. To the air contained in a glass vessel in- 

 verted over mercury a known quantity of carbonic acid is added. 

 The plant previously placed in the receptacle decomposes a part 

 of this, and after a given time the amount decomposed is ascer- 

 tained bv measurement of the carbonic acid that remains. 



\t 



Effects of different gases upon assimilation. A few plants 

 and two or three small Wardian cases, or, better, capacious bell- 

 jars, will answer for this stud}'. Select only sound plants for 

 examination, and be careful to have those in one bell-jar as nearly 

 as possible of the same size and strength as those in the others. 

 Let the air in one of the jars be ordinaiy atmospheric air ; to that 

 in the others add a known but small quantity of one of the fol- 

 lowing gases ; namely, (1) common coal gas ; (2) sulphurous acid ; 

 (3) chlorine. Compare the growth and vigor of the plants from 

 time to time, and observe whether insolation makes any difference 

 in the appearance of the plants exposed to the gases mentioned. 



1 In all cases where an additional amount of gas is introduced into a bell- 

 jar, allowance must be made in some way for the possible increase of pressure. 

 For the necessary correction in these cases, and for other details regarding the 

 management of gases, consult Bunsen's " Gasometry." 



