the bark. For this purpose, circular incisions were made round a
number of young healthy trees, and the bark removed for the space
of about half an inch, or more, of the whole circumference. This, it
was found, by no means impeded the growth of the upper part of the
tree; but, on the other hand, the part of the stem below the incision
scarcely grew at all, and in time even seemed to wither. From the
whole of this investigation it seems probable, that the current of sap
which adds the annual layers of wood to the stem, so far from
ascending, actually descends from the young branches and leaves
through the bark. The branches and leaves which supply this fluid
became hence the next objects of the inquiry.
The conjecture just now mentioned was here confirmed by the circumstance, that when a branch or leaf was left between two cir cular incisions, it continued to receive its nourishment as usual; and the bark under it gave evident marks of increasing vegetation, while that above was not only stationary, but seemed even to decay.
It became now necessary to investigate by what channels leaves receive their nourishment. Some annual shoots were cut from trees, and placed in a coloured infusion. Although this fiuid, it was ob- served, certainly rose into the leaves, yet neither the bark nor the medulla was sensibly tinged by it; but in the centre of the stalks of the leaves were found several bundles of tubes which had been manifestly coloured, and must hence have been the channels of com munication. These tubes were surrounded by others, which, being traced downwards, were found to enter the inner bark, and by no means to communicate with the tubes of the wood: these being colourless, it may reasonably be concluded that they convey a dif- ferent fluid from that which ascends into the leaf.
To the former, or internal tubes, which had not yet been distin- guished by any name, the author thinks fit to assign the appellation of Central Vessels. He then mentions certain spiral tubes which are everywhere appendent to these vessels, and seem to proceed from the sides of the medulla to the leaf-stalk. Particular attention is then paid to the action of the medulla. By extracting parts of it out of the stems of trees, so as completely to interrupt its continuity, it was proved beyond a doubt that it is nowise necessary for the pro- gression of the sap, the tree growing equally, whether this marrow be or be not continued.
The next set of experiments relates to the fructification: and here central tubes were likewise found in the fruit-stalks, which, there is reason to think, are the nourishing ducts of those productions. Many curious circumstances are here mentioned concerning the in- ternal organization and mode of nutrition of certain fruits, such as apples and pears, for which, as well as for several observations on the error of those who have ascribed the ascent of the sap to capil- lary tubes, or to the sole agency of heat, we must refer to the paper, in order to hasten to the part in which the author points out an agent to which he thinks the mechanical propelling force required may be reasonably ascribed.
