52 EXHALATION. RESPIRATION. 



causes the stomata to open. It has been ascertained that the 

 quantity of water thus exhaled is in proportion to the extent of 

 the leafy surface of the plant, and the number of stomata ; thus, 

 fleshy plants, which have but few stomata, lose very little by 

 aqueous exhalation. 



32. Light, as we have said, has the property of causing the 

 stomaia to open, but these orifices close when the plant is placed 

 in the dark. During the night, plants lose very little by evapora- 

 tion ; and it is known that the best way of preserving a bouquet 

 as fresh as possible, is to put it in an obscure place, or at least 

 shelter it from the light of the sun. 



33. Exhalation is more active in dry warm air, than when the 

 atmosphere is cold and damp ; and it takes place more actively 

 in young leaves, than in those of which the surface has been 

 hardened by age. The water that thus escapes is almost pure, 

 and it is estimated that, under ordinary circumstances, it is equal 

 to about two-thirds of the quantity of liquid absorbed by the 

 roots. Sometimes this exhalation becomes even more abundant 

 than absorption, and causes the death of the plant ; this often 

 happens when we transplant a tree in spring, without taking suf- 

 ficient care to lop the branches, for by taking it from the earth 

 we destroy a great many radicles of the root, and absorption is 

 consequently less active ; in order to proportion the exhalation to 

 this enfeebled absorption, gardeners leave but a small number of 

 leaves on the summit of the stem. 



RESPIRATION. 



34. Plants cannot live when deprived of air, and are, just as 

 much as animals, under the necessity of constant respiration ; 

 but their respiration is carried on in a different manner from that 

 of animals. 



35. All parts of the plant, root, stem, and flowers, as well as 

 the leaves, continually absorb a certain quantity of ox'ygen from 

 the air, which combines with the car'bonous particles of the sap, 

 and thus forms carbo'nic acid ; but this carbo'nic acid is not 

 expelled as in animals, but serves for nutrition. 



[Before we proceed further, let us endeavour to obtain clear notions of the 

 meaning of the words ox'ygf.n and carbo'nic acid. 



32. Why are we recommended to put a bouquet in the dark for preser- 

 vation ? 



33. What condition of the atmosphere is most favourable to exhalation ? 

 What is the character of the water exhaled by plants ? What happens if 

 exhalation is greater than absorption? Why do gardeners carefully lop 

 trees that are transplanted ? 



34. Do plants breathe? 



35. What parts of plants absorb ox'ygen ? What becomes of the ox'ygep 

 absorbed ? What is the use of carbo'nic acid to plants 7 



