150 PHYSIOLOGICAL BOTANY. 



481. The quantity of pure water transpired by plants is immense. A forest makes & 

 damp atmosphere for miles aronnd. Dr. Hales, in A series of instructive experiments in 

 transpiration, ascertained tnat a Sunflower three and a half feet high, with a surface of 

 5,61(5 square inches, transpired from 20 to 30 oz. in twelve hours ; a Cabbage 15 to 25 oz. 

 in the same time equal to the transpiration of a dozen laboring men. 



482. Respiration in plants refers to their relations to the 

 atmosphere. So in animals. These relations are in either case 

 vitally important, as may be shown by placing a small, healthy 

 potted plant (sc. Geranium, Mimosa) under the receiver of an 

 air-pump, and thoroughly exhausting the air. At once every 

 vital process ceases no absorption, no assimilation, no irrita- 

 bility, but speedily decay ensues. A vacuum would be no more 

 fatal to a sparrow. Air is quite as necessary to the one as to 

 the other. 



483. Respiration in plants, or aeration (as sometimes called), 

 consists of all those operations by which the sap is brought into 

 contact with the air or subjected to its influence. It occurs in 

 the intercellular passages, in the spiral vessels everywhere, but 

 especially in the leaves and all other organs which have cliloro- 

 phyl and stomata. Its vital importance is manifested in the vast 

 extent of the respiratory apparatus, consisting of millions of 

 leaves and billions of breathing pores (stomata) and trachcce 

 (vessels) ! 



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 solai 

 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 



