THE LEAF 



175 



Fio. 228. Arrange- 

 ment of apparatus to 



ranged in layers alternating with damp cotton bat- 

 ting or blotting paper (Fig. 228) ; close the jar 

 tightly and leave from 12 to 24 hours in the dark 

 to prevent the action of photosynthesis. Then 

 insert a thermometer and note the rise in tem- 

 perature. If a lighted taper is plunged in, it will 

 quickly be extinguished, showing that respiration 

 has been going on. 



191. Respiration in leaves. We see 



from experiments like the foregoing that 



the leaf, besides carrying on the functions 



of digestion, photosynthesis, and trans- 



piration, is also an active agent in the 



work of respiration. In this function by leaf buds. 



oxygen is used up and carbon dioxide 



given off, just as in the respiration of animals; but the 



process is so slow in plants that it is much more difficult 



to detect than the contrary action in photosynthesis, and is, 



in fact, not perceptible at all while the latter is going on, 



though it does not cease even then. 



But while the leaf is the principal organ of respiration, the 

 process is carried on in other parts of the plant as well, 

 else it could not survive during the leafless months of 

 winter. It appears to be most active at night, but this is 

 only because it is not obscured then, as during the day, by 

 the more active function of photosynthesis. Indeed, it was 

 for a long time supposed that plants " breathed " only at 

 night, and it was thought to be unwholesome to keep them 

 in a bedroom. It is now known, however, that respiration 

 goes on at all times and in all living parts of the plant, but 

 the quantity of oxygen taken in is so small from a hygienic 

 point of view that it may be disregarded. 



192. Distinctions between respiration and photosynthesis. 

 - While these two functions are contrasting and antipodal, 



so to speak, in their action, they are mutually complemen- 

 tary and interdependent, the one manufacturing food and the 

 other using it up, or rather marking the activity of those 



