THE path followed by water moving in the plant is readily determined. If young leafyare cut and placed with the cut ends in water coloured with red ink, reddened veins may be seen in the after an hour or so, if these are held up to the light. The effect is even more obvious if white are present, and the experiment succeeds very well with flowering stems of the white dead nettle. Sometimes, white treated in this way are offered for sale. If the is slit lengthwise, narrow red threads are seen, sometimes uniting, and sometimes passing into the stalks and so making connection with the veins in the . Detailed investigation shows that the dye travels only in the small amount of wood present in the conducting strands; it does not travel in the general mass of soft material composing most of the stem.
If a woody branch is cut and stood in diluted red ink, it is found that the dye travels only in the outermost surface of the wood, showing that it is only the youngest wood that is able to conduct watery solutions. Trees are sometimes killed by making a complete and deep horizontal cut all round the base of the trunk. Such a cut removes the young wood and so prevents the upward flow of water from the, thus speedily causing death to all living parts above it.
It is much less easy to trace the paths along which other substances move in the plant. There is, however, little doubt that some of the food substances manufactured by the plant travel in the young wood, and that others are conveyed in special thin-walled tubes. The tubes are independent of and lie outside the woody strands, but, like them, form a continuous system throughout the body of the plant. Where the stem is thick and woody these sieve tubes, so called because the microscope shows that they are crossed at intervals by small perforated plates, lie in the sheath of living material just inside the bark.
It appears then that the stems of plants have two important functions. They support the leaves and flowers, and they provide means for the transport of substances inside the plant. They form a link between thewhich collect water and mineral substances from the soil, and the leaves which take in carbon dioxide from the air, and, after food has been made in the leaves, they provide for its conveyance to
those parts of the plant where active growth is in progress, and where, very often, little or no food is being made.