RESPIRATION 



217 



than it is being excreted. Still, green parts can be shown to 

 respire if they be kept in darkness. Respiration is always 

 going on in actively-living parts, and in all parts of the plants. 

 The absorption of carbonic acid by the plant only takes place 

 in green parts, and only when the parts are exposed to light. 

 There is another difference between respiration and the assimi- 

 lation of carbon from carbonic acid. The volume of oxygen 

 absorbed is not necessarily equal to the volume of carbonic 

 acid released during respiration — in fact, the plant may for a 

 time respire and evolve carbonic acid, when it is not pro- 

 vided with oxygen. Whereas, when carbonic acid is absorbed 

 in the process of assimilation, an equal volume of oxygen 

 is always released, and no oxygen is evolved unless carbonic 

 acid is absorbed. 



Free oxygen is essential to maintain the active life of a 

 flowering plant. — Oxygen is not only absorbed by a plant, 

 but it is absolutely indispensable. A pretty experiment illus- 

 trates this. Two bottles are obtained. Into one a little water 

 is poured, and into the other pyrogallic acid 

 dissolved in an equal amount of water. Two 

 small moist sponges are selected, and cress 

 seeds sown in the small holes of the sponge. 

 The sponges are now suspended from the 

 under-surface of the corks, which are fitted air- 

 tight into the two bottles (see fig. 247, which 

 shows the complete apparatus). The seedlings 

 soon germinate on the sponge which hangs 

 in the air over the water. On the contrary, 

 the seeds suspended over the pyrogallic acid 

 do not germinate : the pyrogallic acid has absorbed all the 

 oxygen from the air and so prevented growth. If we place a 

 plant in an atmosphere devoid of oxygen, all the movements of 

 the leaves and flowers which are signs of active life cease (see 

 next chapter). We can now explain why land-plants which are 

 too freely watered die. Gardeners say that their roots "rot 

 away." The truth is that the earth becomes saturated with 

 water which drives out the air contained in the soil, and in 

 consequence the roots cannot obtain sufficient oxygen and die 

 of suffocation. 



Conditions affecting respiration. — (i.) Plants respire in 

 light and darkness, (ii.) Oxygen. In the absence of oxygen. 



Fig. 247. 



