The Kinds of Work That Are Done by Plants 85 



air passages and along them and through the stomata to the at- 

 mosphere; and the reader should thus visualize these matters in his 

 imagination (figure 29, a). But here comes an interesting point. 

 Since photosynthesis is dependent upon light while respiration is 

 not, there must evidently exist a certain intensity of light at which 

 the two processes in a leaf exactly balance. At such times the 

 processes use one another's gases, and there is no movement of 

 carbon dioxide or of oxygen either into or out of the leaf (figure 

 29, 6) . Such a balance period must occur every day just after sun- 

 rise and before sunset, and on some very dark days it probably 

 lasts for considerable periods. It is of course by virtue of approx- 

 imation to such a balance that some kinds of plants such as Ferns, 

 if not given too much light, can thrive so well for long periods 

 of time in tightly-closed cases, or masses of red-berried vines 

 (Partridge-berry) can exist all winter in little closed globes on 

 dining-room tables. 



We may now express the important facts of the past few pages 

 in another of our botanical verities, to this effect, — that plants, 

 like animals, respire, and in identical manner, absorbing oxygen 

 and releasing carbon dioxide, throughout all of their living parts. 



In the preceding paragraph I have said that the gases enter 

 through stomata and pass along air passages, but I have given 

 no hint of the forces w^hich impel them. This matter will be taken 

 up fully in the chapter on Absorption, where it will be shown 

 that the gases move along diffusively under action of forces 

 internal to themselves. We need only note here that plants have 

 no system at all for absorbing and expelling large masses of air 

 as animals do by the use of their chest-muscles and lungs, — an 

 operation that is always called breathing. Accordingly, the matter 

 can be stated in this way, — that plants respire, but do not breathe. 



It will be well, at this point, to turn aside for a moment from 

 our main subject to consider some phases of plant respiration 

 which have economic importance. The first is concerned with 

 aeration of soils. Roots, like all other living parts, must respire 



