TRANSPIRATION AND RESPIRATION. 151 



tissues in the absence of the sunshine, and by all other parts 

 (root, flowers, fruit, and germinating seeds) at all times. 



485. Hence it appears that there are two phases of aerial action 

 constantly performed and seemingly opposed to each other. One 

 dependent wholly upon the clear sunshine, in which, by the 

 leaves, etc., C O 2 is absorbed, decomposed, and O returned to the 

 atmosphere ; the other, in which O is 'absorbed, and C O 2 ex- 

 haled, by the leaves in the absence of sunshine, and by all other 

 parts (roots, flowers, etc.) at all times. Both are equally and 

 vitally important. 



486. The former process becomes visible to the eye by the raj id development of chloro- 

 phyl accompanying it the latter by its gradual loss. Hence, during a protracted season 

 of cloudy weather, vegetation grows sensibly paler; but a few hours of sunshine restoies 

 the green to its wonted depth and richness. Hence, also, plants growing in darkness or 

 shade, as potatoes in the cellar, are very pale, and manifest their affinity for light by 

 stretching themselves with famishing eagerness toward the slender sunbeam which gains 

 admittance. Analysis shows structures thus grown to be deficient in carbon. 



487. We may easily repeat the experiments of Saussure and Priestley. Place a quantity 

 of freshly gathered leafy stems under a bell-glass inverted and full of rain-water, and thus 

 expose them to the sunlight. Soon bubbles of pure oxygen gas arise and slowly collect 

 above. Now repeat the experiment with boiled or distilled water, and no oxygen will 



.appear. Rain-water contains C0 2 in solution; boiled water does not. The O must 

 therefore have come from the CO a as would appear. 



488. If we enclose, air-tight, in a glass globe the end of a leafy branch, without severing 

 it from the tree, it will be found by careful analysis, after a day of sunshine, that the pro- 

 portion of O has increased at the expense of CO 2 within the globe; and vice versa by 

 night or in the shade. 



489. The results of both transpiration and respiration, as con- 

 cerns the plant, tend to concentrate the diluted sap by the elim- 

 ination of the water, which served merely for its conveyance, and 

 to assimilate it into food capable of being organized into cells 

 and their various contents. And it is proper in this place also 

 to notice the effects of this vast machinery upon the constitution 

 of the atmosphere and its relation to the animal kingdom. 



490. Carbonic acid gas is dissolved in the atmosphere and 

 somewhat uniformly diffused throughout its whole extent, in the 

 proportion of about 4 parts in 10,000, or 22 *g . This gas flows, 

 and is ever flowing into the air from decaying animal and vege- 

 table substances, from combustion, and from the breath of all 

 living animals. The quantity thus added to the atmosphere an* 

 imally is estimated at 100 billions Ibs., or nearly one-tenth of the 

 whole amount of carbon, and yet it does not accumulate. Now 

 if we were able to compute in pounds the annual growth of the 



