177 



soil, nor from decayed leaves, but from the living 

 leaf itself. In what manner, then, is the living leaf 

 enabled to exercise this important function ? The 

 sap of plants, which is chiefly taken up by the roots, 

 passes through the vessels of the stem to the surface 

 and extremities of the leaf, where it suffers conside- 

 rable changes (36. ), and is then again returned by 

 the vessels of the footstalk to the branches and stem. 

 By this flow of the sap, the function of perspiration 

 in the leaf is supported, and when, from any cause, 

 it diminishes, vegetation seems, in a great degree, to 

 be suspended. This happer.s to many plants during 

 a cold rain, as well as through the night ; and the 

 leaves, in consequence, assume a change of form 

 and condition, which some have supposed to resem- 

 ble a state of sleep ; and in winter, we know, with 

 the cessation of the flow of the sap, vegetation whol- 

 ly ceases. But the plant, during all these times, is 

 in contact with the atmosphere, and the cessation of 

 the usual vegetative changes cannot, therefore, be 

 attributed to the want of oxygen gas : they must, 

 consequently, proceed from some alteration in the 

 state of the plant itself, which alteration consists in 

 the diminished or suppressed flow of the sap, by 

 which all the vegetable secretions are suspended. 



140. If, farther, the leaves of a plant be separated 

 from the stem, they retain life for a considerable 

 time : but the circulation of fluids through them 

 must then of necessity be cut off. Dr Woodhouse 

 exposed handfulls of the leaves of twelve different 

 plants separately to the light of the sun in 40 ounce 

 measures of air for four hours, and its purity was 

 found to be neither increased nor diminished. Af- 



M 



