156 RESPIRATION. 



TRANSPIRATION. 



808. Transpiration relates to that important office 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 the activity of 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 quantity of pure 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 5,61G 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. 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 bell-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. 



RESPIRATION. 



812. Respiration in plants refers to their refations to the atmos- 

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

 tant. 



813. Experiment. 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 assimilation, no irritability, but speedily do- 

 cay ensues. A vacuum would be 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. Respiration 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 tracheae (vessels) ! 



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

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

 etc., are these : 



