RESPIRATION. 157 



1. Carbonic acid (C 0. 2 ) is absorbed by the leaves and all green tis- 

 sues, 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 ger- 

 minating seeds at all times. 



3. The oxygen thus absorbed unites with some of the free (or nas- 

 cent) carbon already in the tissues, and forms carbonic acid. 



4. By a process of assimilation (§ 747) 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 tissuea 

 in the absence of the sunshine, and by all other parts (root, flower?, 

 fruit, and germinating seeds) at all times. Hence it appears that there 

 are 



818. Two phases of aerial action constantly performed and seem- 

 ingly opposed to each other. One dependent wholly upon the clear 

 sunshine, in which, by the leaves, etc., C 2 is absorbed, decomposed, and 

 returned to the atmosphere ; the other, in which O is absorbed, and 

 C 0, exhaled, 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. 



819. The former process becomes visible to the eye by the rapid development 

 of chlorophylle 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 restores the green to its wonted depth and richness. 



820. Blanched plants. Hence, also, plants growing in constant darkness and 

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

 by stretching themselves with famishing eagerness towards the slender sunbeam 

 which gains admittance. Analysis shows structures thus grown to be deficient in 

 carbon. Wo may easily repeat thp 



821. Experiment of saussure. Place a quantity of freshly gathered leafy stems 

 under a bell-glass fall of rain-water, and thus expose them to the sun. Soon bub- 

 bles of gas arise and slowly collect above, pure oxygen gas, as long ago proved by 

 Dr. Priestly. 



822. Repeat the experiment with boiled or distilled water, and no oxygen will 

 appear. Rain-water contains C Oj in solution, boiled water does not. The must 

 therefore have come from the C 2 as would appear. 



823. Experiment. Inclose air-tight in a glass globe the end of a leafy branch, 

 without severing it from the tree. Thus it has been found by careful analysis after 

 a day of sunshine that the proportion of O was increased at the expenso of C O a 

 within the globe ; and vice versa by night or in the shade. 



824. The results of both transpiration and respiration, as concerns 

 the plants, tend to concentrate the diluted sap by the elimination of the 



