RESPIRATION. 31 



XVI. RESPIRATION. 



The measure of this process is usually found in the amount 

 of carbonic acid given off by plants. The methods of deter- 

 mination of this amount are, although apparently simple, open 

 to some objections ; but by the exercise of great care in the 

 management of the simple appliances, their results are in gen- 

 eral trustworthy. 



The carbonic acid which is given off by the plant may be 

 measured in one of the two following ways : (1) A current of 

 air freed from all its carbonic acid by means of wash-bottles con- 

 taining potassic hydrate is allowed to pass into a receptacle in 

 which are confined the plants to be examined. The air with- 

 drawn from this receptacle passes slowly through Liebig's potash 

 bulbs in which are held a known amount of potassic hydrate. 

 At the conclusion of the observation the amount of carbonic acid 

 which has been given off by the plants and been taken up by the 

 potassic hydrate in the bulbs can be accurately determined. 

 (2) The current of air which is withdrawn from the receptacle 

 containing the plant is permitted to pass very slowly through a 

 long slightly inclined tube in which is held a solution of pure 

 baric hydrate. As the bubbles of gas pass through this liquid 

 and give up their carbonic acid, they cause an abundant precipi- 

 tation of baric carbonate in it. The second method, which is 

 essentially that of Pettenkofer, yields uniform results, and is 

 in general to be preferred to the first. It is better applicable 

 to observations upon intramolecular respiration ; in which, as 

 pointed out in 981, some gas like nitrogen or hydrogen, wholly 

 free from any trace of oxygen, is allowed to come in contact with 

 plants or parts of plants, and the amount of carbonic acid given 

 off is determined as in the former case. Interesting results are 

 obtained by placing in the receptacle very young seedlings, or 

 buds which have just begun to unfold. 



XVII. GBOWTH. 



The measnrement of growth. Growth can be satisfactorily 

 measured in the three following ways, each of which is adapted 

 to particular instances : 



1. Direct measurement. Determine the place and rate of 

 growth of young internodes of any rapidly developing plant, for 

 instance Morning Glory, by marking the whole space of the 

 internodes into equal intervals, and subsequently determining 



