TRANSPIRATION AND RESPIRATION. 151 



484. The facts connected with respiration, which seem to have 

 been well established by the experiments of Saussure, Garreau, 

 Moue, Draper, etc., are these : 1. Carbonic acid (C O 2 ) is ab- 

 sorbed by the leaves and all green tissues, under the direct solar 

 light. 



2. Oxygen (O) is absorbed by the leaves and all green tissues 

 in the absence of direct solar light, and by the roots, flowers, 

 fruits, and germinating seeds at all times. 



3. The oxygen thus absorbed unites with some of the free 

 (or nascent) carbon already in the tissues, and forms carbonic 

 acid. 



4. By a process of assimilation ( 439), carbonic acid within 

 the green tissues, from whatever source derived, is decomposed 

 under the direct sunshine, and its carbon is retained ; but, 



5. Its oxygen is set free and exhaled. 



6. Carbonic acid is exhaled by the leaves and all the green 

 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 a is absorbed, decomposed, and O returned to the 

 atmosphere ; the other, in which O is absorbed, and C O a 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, daring a protracted season 

 of cloudy \veather, vegetation grows sensibly paler; but a few hours of sunshine restores 

 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 thu? 

 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 CO., in solution; boiled water does not. The O must 

 therefore have come from the CO 3 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 a within the globe ; and vice vtrta by 

 ni^ht or in the ehade. 



