46 HOFFMANN ON THE CIRCULATION 



that the fluid had ascended in the prosenchyma accompanying 

 the vessels, but not in these themselves. 



From these and similar experiments it follows that in the ab- 

 sorption of fluids through the leaves, they are conveyed most 

 readily by the tracheae or the prosenchyma closely surrounding 

 these; in plants gorged with sap more readily in the latter, and 

 the reverse in dry woody plants. But even in the most succu- 

 lent vegetables, only a somewhat longer continuance of the in- 

 troduction of the fluid, or a greater quantity of it, is requisite to 

 cause it to pass very readily into the air-tubes, and at length 

 into all parts. It would therefore be erroneous to assume that 

 any particular anatomical system is exclusively charged with the 

 conveyance of unelaborated fluids in the ascending or descending 

 direction. It was above all seen, that the tracheae do usually 

 convey air in summer, but very readily become temporarily 

 more or less, or even wholly filled with fluids which displace the 

 air. In fact, chemical reasons led me to consider the existence 

 of the gas in the spiral vessels and spiroids as nothing more 

 than a result of the absorption of crude fluids from the soil, which, 

 ascending in the higher and warmer layers of the plant, at once 

 give off almost unaltered the gases dissolved in them, these being 

 diffused through those communicating passages, and so gra- 

 dually evaporated outwards and upwards without doing any 

 mischief. In this point of view the vessels would be regarded 

 as ' tubes of safety.^ 



It merits some attention, that, as the last experiments prove, 

 no parts take so little share in the conduction of the solution 

 downward as the layers of the bark. I therefore took occasion 

 to investigate the capability of the bark to convey fluids by a 

 direct experiment. This was done by removing every other 

 passage but the bark from the descending fluid. 



Salia; vitellina. — On the 1 9th of June a fresh pendent twig 

 1^ line in diameter, had the bark slit up for the length of 1 inch 

 at the side, 5 inches from the end ; the bark was turned back 

 and the wood within completely removed for a length of 2 lines ; 

 then the bark was returned into its place, rolled up in a living 

 leaf to prevent drying, and the shoot strengthened by a splint. 

 Lastly, a leaf situated below the excised wood was immersed in 

 the solution. After twenty-four hours the fluid had advanced 



