150 PHYSIOLOGICAL BOTANY. 



CHAPTER XL 



TRANSPIRATION AND RESPIRATION. 



479. Transpiraiion relates to that important office per- 

 formed by the leaves and other green organs." whereby pure 

 water is separated from the crude sap and given oif into the air. 

 It takes place chiefly through the stomata, and is greatest by 

 day, and in a warm, dry atmosphere. 



480. Upon the activity of transpiration depends also the 

 amount of absorption. It not only makes room for the fluids 

 from below to enter, but by disturbing their equilibrium it 

 creates an upward tendency, as the flame of a lamp draws the 

 fluid up the wick. All the mineral and organic constituents of 

 the sap are of course left behind, in the plant. 



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

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

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

 5,616 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 chloro- 

 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 tracheae 

 (vessels) I 



