MOVEMENT OF WATER IN THE PLANT BODY The water that enters the plant from the soil ultimately escapes from the leaf surface passing through the stomatal pores in the form of water vapour. The question arises as to the path taken by this water. It has been known a long time that the main body of the water passing through the vascular plants travels by way of the xylem, i.e., the wood. (See above, Anatomy.) The evidence for this is various. It is found that if the trunk of a tree is "ringed," i.e., cut through right down to the wood throughout the circumfer ence of the tree, water continues to pass up the tree for a long time and apparently unhindered. Again if cut shoots are placed in a solution of a dye such as eosin, the wood alone is deeply stained showing that the dye solution has travelled up through this tissue. The frequent occurrence in the wood of wide open tubes con taining no protoplasm also indicate its function as a conducting channel.
The root as anatomical studies show, usually exhibits elongated outgrowths from certain of its superficial cells. These outgrowths are known as root-hairs and serve to increase the surface of the root available for absorp tion. These hairs are in contact with the films of water on the soil particles and they can withdraw water from the soil if their suction pressure (see above) is greater than the imbibition forces of the soil, i.e., the forces tending to retain the water in the soil. The water thus absorbed, if it is to reach the leaves, must pass across the root to the xylem strands and the question arises as to the mechanism of this transport. It has been shown by the work of Ursprung and Blum that there is a gradient of increasing pressure from the root hair to the endodermis; by this means the water will pass from cell to cell across the cortex. At the endodermis there appears to be some abnormal condition for in the absorbing root of the broad bean it is found that while the suction pressure rises from outside to the sixth cortical cell from 0.7 to 3.o atmospheres, yet at the endodermis it falls to 1•7 atmospheres; as a result the direction of movement of the water should be reversed and the water pass back again; this cannot be the case so the matter requires further investigation.
That the water rises in the wood to supply the needs of transpiration is well established, but the mechanism of this rise has been much disputed. It is clear that any explanation must meet the extreme case of the tall leafy tree. Now a single tree may lose many gallons of water a day in transpiration and a blue gum tree (Eucalyptus) of Aus tralia or a Sequoia of N. America may reach a height of 30o ft. or more. A mechanism has thus to be found which will not merely hold up a column of water at a height of 30o ft.—this is merely a problem in statics—but one that will drive (or pull) many gal lons of water a day to this height. Many different forces have been invoked to explain the rise. The force of capillarity has been called in but measurements of the size of the wood vessels show that capillarity will not explain the rise of water to the height of a tall tree, let alone the continued movement of the water upward. Then again atmosphere pressure has been suggested as concerned in the movement, but such a pressure would only explain the rise of water to a height of a little over 3o ft. "Root pressure" has been called in as the motive power for the rise of the transpiration stream. When the stem of a wood plant is cut across near the ground a flow of sap under considerable pressure may occur from the wood of the cut stump ; the pressure bringing about this flow is known as root pressure. Unfortunately for this explanation no pressures have been observed which are adequate to explain the rise of water to the top of a tall tree. Again root pressure is found most active in the spring, while the transpiration stream flows most actively in the summer. Then again we have the so-called vital theories of the cause of the ascent of sap in all trees. In these theories the living parenchyma cells which are so charac teristic of the wood are supposed to exert a pumping action. They are assumed to take water from a lower wood vessel and pump it into a higher vessel with which they are in contact.