30 THE WORK OF THE LEAF [CHAP. 



though, of course, with leaves and other green parts the 

 jar must be shielded from the light. Respiration is 

 most active in the flower, and if the jar is filled with a 

 lot of young, active tropseolum flowers, a very rapid 

 evolution of carbon dioxide can be observed. Respira- 

 tion is always going on, though it is reduced to a 

 minimum at temperatures below 40° F. ; it is, in fact, a 

 process essential to the life of all the living cells that 

 make up either plants or animals. In a leaf, however, 

 it is never so rapid as the opposite process of 

 assimilation is in the light ; were not assimilation very 

 much more active than respiration, the plant would 

 never increase in weight at all. It is due to the 

 respiration, which is a necessary accompaniment of their 

 life, that roots, like mangolds or potatoes, gradually lose 

 weight when they are stored, for the loss is not only of 

 water but of dry matter also. Bulbs of tulips or 

 daffodils are also noticeably lighter in the autumn 

 when they are planted than in the early summer when 

 they are first put into store, and if the planting be 

 deferred for any reason the loss of weight becomes even 

 more evident 



Another experiment may be carried out to illustrate 

 further the processes of respiration and assimilation : 

 About a hundred barley grains are soaked and placed 

 with a little water in a stoppered bottle holding about a 

 pint, the stopper is left out and the bottle exposed to 

 the light until the barley has shot and made a fair 

 amount of leaf. The stopper is then inserted, and the 

 bottle is put away in a dark cupboard or drawer for 

 two days ; on taking it out and testing the air in the 

 bottle by means of a lighted taper, the taper will 

 be extinguished. In the dark respiration only has been 

 going on, and so much carbon dioxide has been pro- 

 duced and so much oxygen used up that the air in the 



