156 BESPIRATION. 



TKANSPIRATION. 



808. Transpiration relates to that important oiSce performed by the 

 leaves and other green organs, whereby pure water is separated from the 

 crude sap and given off into the air. It takes place chiefly through the 

 stomata, and is greatest by day and in a warm, dry atmosphere. 



809. Upon THBi|A.CTiviTY or transpiration depends also the 

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

 low 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 in the 

 plant. 



810. The quantitt op pueb water transpired by plants is immense. A 

 forest makes a damp atmosphere for miles around. Dr. Hales, in a series of instruc- 

 tive experiments in transpiration, ascertained that a sunflower three and a half feet 

 high, with a surface of 5,616 square inches, transpired from 20 to 30 oz. in twelve 

 houi-s; a cabbage, 15 to 25 oz. in the same time — equal to the transpiration of a 

 dozen laboring men. "We may easily " 



811. Experiment with a single leaf recently plucked, say of Podophyllum. In- 

 sert its petiole in a narrow-mouthed goblet of water, and around it fill the mouth 

 with dry cotton to restrain evaporation. Over the whole place a beU-glass and ex- 

 pose to the sunshine. The vapor transpired will condense on the bell-glass, equal- 

 ing (save the solid matters) the loss in the goblet. 



KESPIRATION. 



812. Ekspiration in plants refers to their reMions to the atmos- 

 phere. So in animals. These relations are in either case vitally impor- 

 tant. 



813. EzPERiMENT. Place a small, healthy potted plant (sc. Geranium, Mimosa) 

 under the receiver of an air-pump, and thoroughly exhaust the air. At once every 

 vital process ceases — no absorption, no assimilalion, no irritability, but speedily de- 

 cay ensues. A vacuum would bo no more fatal to a sparrow. Air is quite as 

 necessary to the one as to the other. 



814. Illustration. So also when only the roots are excluded from the air by 

 being buried deeply in an embankment, the tree suffers injury and perhaps perishes. 



815. Kespibation in plants, or aeration (as sometimes called) 

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

 tact with the air or subjected to its influence. It occurs in the inter- 

 cellular passages, in the spiral vessels everywhere, but especially in the 

 leaves and all other organs which have chlorophylle and stomata. 



816. The vital importance of respiration is seen in the vast ex- 

 tent of the respiratory apparatus, consisting of millions of leaves and 

 billions of breathing pores (stomata) and trachese (vessels) ! 



817. The pacts connected with respiration, which seem to have been 

 well estabhshed by the experiments of Saussure, Garreau, Moue, Draper, 

 etc., are these : 



