u.] CARBON DIOXIDE SPLIT UP BY PLANTS 25 



in bright light this breathing process is masked without 

 being stopped, by the reverse process of taking in 

 carbon which we are now considering. The method of 

 experimenting we have just been describing has been so 

 modified as to become a means of measuring exactly 

 the powers of the leaf in splitting up carbon dioxide 

 under various regulated conditions. In Dr Horace 

 Brown's experiments, a leaf still attached to the plant is 

 enclosed in a glass-sided box and a current of air is 

 drawn through the box, the proportion of carbon 

 dioxide being determined in the air before it reaches 

 the leaf and again when it leaves the box. Working in 

 this fashion, Dr Brown has shown that a square 

 decimetre (4 inches square) of active green leaf will 

 decompose about O'OoS gramme per hour of carbon 

 dioxide in ordinary diffuse daylight, which amounts 

 correspond to a formation within the leaf of about o-oo6 

 gramme of dry matter. The light may be diminished 

 considerably without any corresponding falling off in 

 the power of the leaf, until a certain point is reached 

 beyond which the action will fall off in proportion to the 

 reduction in the light. It is easy to see how dependent 

 plants are upon light by the failure of most of them to 

 flourish or even to grow in anything approaching dense 

 shade ; only a few classes of green plants, like ferns and 

 mosses and those generally of a very slow-growing 

 character, have adapted themselves to live in compara- 

 tive shade, whereas all farm crops, which are of course 

 distinguished by the enormous quantities of dry matter 

 they manufacture, must be exposed to the fullest possible 

 light. The process of fixing the carbon of carbon 

 dioxide and giving off oxygen by the green leaf in sun- 

 light, which we shall in future call assimilation, may now 

 be studied by a different method, depending upon the 

 fact that the first visible product of the action in the 



