244 PHOTOSYNTHESIS 



Without such control, determinations of the quantities of carbon dioxide 

 absorbed or of oxygen emitted are of very Httle vakie. In order to main- 

 tain these controlled conditions the amount of plant material or the 

 size of the plant that can be used is naturally limited. It has, for in- 

 stance, been found impractical thus far to determine the rate of gaseous 

 exchange of a small tree though such determinations would undoubtedly 

 be very desirable. To maintain a truly equable temperature in an en- 

 closed space sufficiently large to contain a tree, while at the same time 

 it is being illuminated, requires rather elaborate apparatus. Moreover, 

 under such conditions the position of the leaves is apt to change so that 

 it is impossible to obtain the rate of photosynthesis per unit of illuminated 

 leaf surface. To this may be added the difficulty that in a large space 

 changes in composition of the atmosphere are relatively slight which by 

 the ordinary methods of gas analysis can be detected only with uncer- 

 tainty. Finally, if it is desired to obtain information regarding the 

 photosynthetic process under natural conditions, i.e. using atmospheric 

 air with a carbon dioxide content of approximately 0.03 per cent by 

 volume, large volumes of air must be passed over the plant. If this is 

 not done, the carbon dioxide content of the air will be so greatly re- 

 duced that the rate of photosynthetic activity will be greatly impaired 

 and moreover, natural conditions will not be maintained. Thus for ex- 

 ample McLean ^" had to use a flow of nearly 19 liters per hour to deter- 

 mine the rate of photosynthesis of a single sugar-cane leaf. The dift'erences 

 in the carbon dioxide-content of the air stream before and after passing 

 over the leaf are very small., For this reason only the most exact methods 

 of gas analysis can be used. 



In much of the earlier work on photosynthesis, the plant was enclosed 

 in a vessel in an atmosphere containing a known quantity of carbon 

 dioxide mixed with air. After exposure to light for a time, the gas in 

 the container was again analyzed ; the reduction in the amount of carbon 

 dioxide and the increase in that of oxygen was ascribed to photosyn- 

 thesis. This principle has its decided limitations especially when it is 

 desired to determine rates of photosynthesis, for the rate is greatly in- 

 fluenced by the partial pressure of the gas, which is by this method 

 constantly being reduced in the course of the experiment. For this and 

 other reasons the method just mentioned has been replaced by that of 

 keeping the plant in a stream of air, the composition of which is deter- 

 mined before and after passing over the plant. The simplest method of 

 making such determinations is to absorb all the carbon dioxide in the 

 air stream by means of barium hydroxide or sodium hydroxide ; but it 

 is frequently desirable to determine the oxygen as well as the carbon 

 dioxide. Where gas samples are taken from an air stream the best 

 method is to make accurate gas analyses. 



One of the most convenient forms of gas analysis apparatus is the 



'= McLean, F. T., Ann. Bot., 34, 367 (1920). 



