THE METABOLISM OF PLANTS. 



253 



exposure to darkness must, therefore, eventually prove fatal 

 to the plant, the length of the time being determined by the 

 amount of reserve plastic material which the plant possesses. 

 On the other hand, adequate exposure to light enables the 

 green plant to assimilate its food, and thus not only to make 

 good the loss due to respiration, but to increase in weight. 



The foregoing statements are well illustrated by some experiments of 

 Boussingault and of Sachs. 



i. (Boussingault.) Two beans of known weight were sown in 

 moistened pumice-stone on June 26 ; they were allowed to grow until 

 July 22, the one exposed to daylight, the other in the dark. The weights 

 of the seedlings were then determined. 



Plant in light. 



Weight of seed ... 0-922 grme. 

 seedling... 1*293 



Gain 0-371 



Plant in darkness. 

 Weight of seed . . . 0*926 gone. 

 seedling... 0-566 



Loss 



0-360 



2. Boussingault ascertained with Oleander-leaves that for one 

 square metre of leaf-surface 6336 cub. centim. of carbon dioxide were 

 decomposed during 12 hours' exposure to daylight, whereas only 396 c.c. 

 of carbon dioxide were exhaled during twelve hours' darkness. It is 

 evident from this that the gain in weight during the twelve hours of day- 

 light was greater than the loss during the twelve hours of darkness. 



3. Sachs sowed four seeds of Tropceolum majus in each of ten pots ; 

 the seedlings appeared above the soil on April 28. They were then 

 treated as follows : I. two pots were placed in a dark cupboard ; II. two 

 pots were so placed in a room that they received only diffuse daylight ; 

 III. two pots were so placed in a window that they received diffuse day- 

 light for seven hours daily ; IV. two pots were so placed in a window 

 that they received diffuse daylight and often direct sunlight for about 

 six hours daily ; V. two pots were so placed in a window that they received 

 as much light, both diffuse daylight and direct sunlight!, as possible. On 

 May 22 the weights of the plants in the different pots were determined. 



II. 



I. 



Weight of seeds 0-394 



seedlings 0-238 



Loss 0-156 0*130 0-093 Gain ox>86 o* 



Q'394 

 0*264 



III. 



Q'394 

 0-301 



IV. 



0-480 



V. 



0*394 grme. 

 1-292 



A plant which does not possess chlorophyll is capable, on 

 the contrary, of assimilating its food in the absence of light. 



