CONDUCTION (from Latin conductio, bringing together). In botany, this is the tech nical name for the process whereby water and food-stuffs are transmitted from the roots of a plant to its trunk or stem and leaves. The paths by which these substances traverse the plant are well known. They enter the thin walled root-hairs by osmosis, the food-salts being in a state of aqueous solution. From this point up, there is good evidence that the path of the water and the salts dissolved in it lies through the tracheids and the long tubular ves sels of the fibro-vascular bundles, which are situated in the wood of the roots and stems of the Dicotyledons and Conifers, and form part of the leaf-veins. Only the young wood, how ever, takes part in this upward flow of sap. The motive force in the process has only re cently been understood: the osmotic pressure of the juices of the root unquestionably plays a large part in the matter, but it is alone un able to make the sap rise to the tops of our larger trees. Furthermore, a cut branch with its end dipped in water will still show a flow of sap. Furthermore, if a living twig be di vided under mercury, the wood-vessels will be found to contain some of the mercury on mi croscopical examination. As mercury will not enter a capillary tube of cellulose except under the action of a force, this result indicates that the pressure in the wood-vessels is less than that of the atmosphere. This could not be the case if root pressure were the only agent tend ing to produce the upward flow of sap; there must be accordingly some force producing a suction from above. This force is furnished by the osmotic attraction for water of the leaf cells, whose contents are concentrated by evap oration and the consumption of their water in the photosynthesis of carbohydrates. This ac tion is propagated from cell to cell until it reaches the vessels of the stem, each cell being, to put it loosely, sucked dry by its thirsty pred ecessor. This suction, producing a pressure in
the vessels less than that of the atmosphere, causes the sap to give up part of its dissolved gases. This is probably one of the reasons why the vessels of a plant contain gases, which are, of course, in a state of rarefaction.
It has been shown that the path of the car bohydrates and other substances generated in the leaves is quite different from that of the water which comes up from the roots with its dissolved salts. For example, a girdled twig will keep green if its end is immersed in water, which shows that wood transmits water, but it will send down roots only from points above the injured region, because the portion of stem beneath the girdle has lost its power of ob taining food-material from the leaves and of growing thereby. The bark, therefore, and more specifically the bast layer of the bark, as it has been shown, is the path of the sap. de scending from the leaves. This layer contains the remarkable sieve-cells — elongated cells placed end to end in long chains, with the plates between adjacent cells in these chains perforated with numerous small holes. The mechanism of the transmission of food-stuffs appears to be diffusion.
In addition to the tracheids, vessels and sieve-tubes, we find another system of tubes in the vascular tissue of plants. This system bears the milky juice or latex of such plants as the rubber-tree the dandelion and the milk weed. Though there is evidence that the latex exercises a nutritive function, the nature of this is not thoroughly known. Consult Timi riazeff, C. A., The Life of the Plant' (trans. by A. Cheremetieff, London 1912).