92 Mr. Burnett on the Development 



the sap might first have exuded from the upper side of the 

 perforation and have collected on the lower, I preferred notches 

 to holes in my experiments, and furthermore had a diaphragm 

 made in the centre of the wound, thus separating the upper 

 and under parts, so that this shelf would intercept the descent 

 of any sap to the lower section ; and the moisture still exuding 

 and covering the lowest part, while the upper continued dry 

 or nearly so, very satisfactorily demonstrated the upward pro- 

 gress of the sap. Again, in other experiments, in which vines, 

 rose-trees, &c., were cut off within a few inches of the ground, 

 the sap exuded abundantly from the stumps, although there 

 were in these experiments no stems or branches present from 

 which it could possibly descend. 



As a converse to these experiments, I repeated some of those 

 of Bonnet, placing vine-leaves, &c., on the surface of water 

 by their different surfaces, but always taking care, which he 

 seems scarcely to have done, that the cut ends of the petioles 

 were covered with soft wax, that no fluid might enter thereby, 

 but all that was absorbed must have passed through the cuti- 

 cular covering of the organs : and leaves thus treated will 

 continue fresh and green for days, nay even for weeks, and 

 some, if wholly immersed, for months together; while other 

 experiments shew that, if left without water in a dry place, 

 they wither and decay in a few hours. Again, that these 

 observations might not be open to the same objections which 

 have been raised to somewhat similar ones, viz. that although 

 it is confessed leaves and plants continue fresh longer when 

 placed on water or in damp situations, than when left without 

 it and in dry ones, yet that this arises from the moisture with 

 which the specimens are surrounded preventing the exhalation 

 of the peculiar juices of the plant, rather than from any actual 

 intus-susception of external fluid which the leaves effect, the 

 following modification of these experiments was devised. To 

 decide this point, I took several leaves of Potamogeton natans, 

 which, when wiped quite dry, were weighed, and after remain- 

 ing out of water for two hours they lost from three grains and 

 a half to five grains and a quarter each ; they were then put in 

 water, and after the lapse of two hours more were again wiped 

 quite dry and again weighed, by which it was proved that 



