33 



26. But although water and heat alone be the 

 first requisites to the commencement of vegetation, 

 yet they are not of themselves sufficient to carry on 

 that process. Air likewise is required. Mr Papin 

 put an entire plant into the exhausted receiver of an 

 air-pump, and it soon perished ; but on keeping the 

 whole plant, except the leaves, which were exposed 

 to the air, in this vacuum, it continued to live a long 

 time *. ' Dr Hales, Du Hamel, and Mr Knight, 

 found that plants do not vegetate when deprived of 

 their leaves : and it is well known, that when the 

 leaves of a plant or tree are destroyed by insects, its 

 vegetation for that season is suspended, and the fruit 

 gradually decays. We have already seen, that when 

 the upper surface of the leaf was covered with var- 

 nish or laid on water (23.) it died in a few days ; and 

 Dr Darwin smeared the upper surfaces of the leaves 

 over with oil, and the plant died in a day or two af- 

 ter f. As thus without air plants do not vegetate, 

 and as they cease also to grow when deprived of 

 their leaves, even although air be supplied, we must 

 conclude, not only that air is essential to the growth 

 of plants, but that their leaves are the organs which 

 act upon this air. And farther, as their death takes 

 place if the upper surfaces cf their leaves be smear- 

 ed over with varnish or oil, we must also conclude, 

 that it is to some action going on between those sur- 

 faces and the air that the continuance of their life is 

 to be ascribed. Let us next then inquire into the 



* Darwin's Phytologia, p. 51. f Ibid. p. 45< 



c 



