29 



nish, and the former, in a given time, suffered little 

 diminution of weight, but the latter became much 

 lighter. Similar leaves also, were laid on the sur- 

 face of water, and those which had their superior 

 surface inverted gained little weight, and for the 

 most part died in a few days ; while such as had 

 their inferior surface applied to the water, became 

 much heavier, and flourished many months. These 

 facts make it evident, that perspiration and absorp- 

 tion are not performed by the same vessels, but that 

 each has its peculiar organs *. The office of ab- 

 sorption is performed by fine hairs or points, placed 

 on the under surface of the leaves, which in many 

 plants are hollow tubes constructed for that purpose. 

 When leaves have no such points, small apertures 

 are found in their place t- The leaves of plants be- 

 come gradually less and less fit for carrying on these 

 functions ; not so much, however, from any decay 

 or inaptitude in their structure, as from the decli- 

 ning influence of another agent, whose operation we 

 are next to consider. 



24. The fluids of plants which are taken up by 

 the roots, and afterwards perspired by the leaves, 

 require, for their due transmission through the stem, 

 a certain degree of heat : hence they flow most free- 

 ly in the day and in sunshine, and are checked du- 

 ring the night and in frost. Dr Hales observes, that 

 during moisture and warmth, the flow of the sap is 

 most vigorous, but that a cold easterly wind imme- 



* Dr Bell, Manchester Memoirs, vol. ii. p. 411. 

 f Wildenovv's Principles of Botany, p. 298. 



