336 OS THE STEM Or TREES. 



another; and its virtues, if I may so express myself. The 

 bark is generally green, the inner bark white, yellow, or 

 green. The former consists of vessels crossing each other; 

 the latter of bundles of vessels of two sizes, the large ones 

 being formed in a very peculiar manner. They consist of 

 broad cylinders, having a bottom with a hole in it, through 

 which the liquid passes, though not with perfect ease. On 

 exposing several pieces of the inner bark to the solar 

 Extraordinary microscope, the moment I turned the light on the specimen, 

 liquid ^ the juice of which had before proceeded up the pipes rather 

 slowly, it was suddenly propelled forward with a force 

 truly astonishing. When I increased the heat and light by 

 pointing the full focus of the rays on the vessels, the power 

 of the heat was too strong, and broke through the side divi- 

 sions, inundating the specimen : but when I merely kept up 

 a proper degree of light and heat, it was curious to observe 

 the liquid pass from pipe to pipe, in one regular and easy 

 flow ; making a little stop as it issued through the straitened 

 apertures at the bottom of the vessels. I have often stood 

 more than an hour watching the current, (which passes 

 however much slower than the sap does) nor could I per- 

 ceive, that it required (while the heat and light were on it) 

 any additional expedient to hasten it; but in the night, 

 when both are wanting, the pressure Mr. Knight mentions 

 from the bastard grain is (I should suppose) very likely to 

 assist or quicken its flow ; and as at night it is pressed 

 against the cylinders, it is at this time (I should conceive) 

 it would have its effect. This part is however formed in the 

 , wood only; but the contraction at the bottom of the large 

 vessels of the inner bark, it is probable may serve the same 

 purpose, that of forcing the liquid forward, by lessening 

 the apertures, and giving therefore more impetus to the 

 Curious forma- current. The vessels are also of great thickness in pro- 

 vessels in the portion to their size ; and have on them a peculiar circular 

 bark. thing resembling a cullender full of very diminutive holes, 



so small that no liquid could pass them; but in viewing the 

 th^t' 3 ° f d' r ^ick juice, that runs through these pipes, I observed many 

 bubbles of air, which, as the heat increased or diminished 

 their size, accelerated or retarded the flow of the liquid. 

 Might not these apertures be designed for the entrance of 



air 



