Movement of Water in the Plant Body

The objections to these vital theories are mainly twofold; they are inconsistent with experimental data and also with the known structure of the wood. The strongest experimental evidence against vital theories is obtained from the results of poisoning experiments. E. Strasburger carried out in 1893 some classical experiments with an oak tree over 7o ft. high and with tall climbers like wistaria and hop. The oak was cut off at the base and the cut end placed in a tub of picric acid, a highly poisonous yellow solution, which was found to be taken up in considerable quantities. After three days in this fluid the trunk was placed in a solution of a reddish dye (fuchsin) and there left for another six days. The oak trunk was then split down the middle when it was found that the yellow poison had been drawn up to 7o ft., and among the yellow stained wood were red patches showing that the dye had been drawn up after the picric acid, i.e., through the dead wood. In the case of the two climbers lengths of 3o-4o ft. were killed by heat when it was found that, as in the oak, coloured solutions were drawn up through the stems in spite of the killing of the living cells. These experiments seem to show clearly that fluids will rise in the wood of tall plants without the intervention of the living cells of the wood.

Cohesion Theory of the Ascent of Sap.

It is evident that in the absence of a pumping mechanism in the wood itself the water rising must either be pushed from below or pulled from above. Since water will rise even in tall trees when they are severed at the base some pulling force from above would appear to be acting and it can be demonstrated that a cut leafy shoot does exert a pulling or sucking force on a column of water. If a glass tube is fixed with an airtight junction to the lower end of a small leafy branch of a tree (the wood being bared at the point of junction), water will naturally rise in the tube. If the tube is filled with boiled water and the lower end placed in a dish of mercury, the mercury will be drawn up after the water. Under favourable conditions when no air bubbles develop in the system the mercury column will be raised to a height above that of the barometric column. This demonstrates that the cut branch exerts a definite sucking force on the water column. The same fact could be demonstrated without mercury using water alone in the tube, if the space above the water in which the tube dipped was exhausted of air. More elaborate experiments of this type show that a transpiring branch will exert a pull of 7 to 8 at mospheres. Such experiments demonstrate not only the sucking force exerted by a cut leafy shoot but also a peculiar physical property of a fluid such as water and mercury, namely, its co hesion or tensile strength. If a column of water or mercury is held up in a vertical tube without being supported from below, it must evidently be held by a pull from above. If, however, the fluid is transmitting a pull its particles must cohere together like the particles of a steel wire. Now it is found that if a fluid, like water, be free from air bubbles and is enclosed in a rigid tube to the wall of which the fluid adheres, it can then transmit a very considerable tension, owing to the attraction of the water particles for one another which is spoken of as their cohesion. Under con ditions spoken of, with the fluid adhering to the walls of a rigid tube, the fluid is unable to change its shape and hence, as the result of its cohesion, is able to transmit a pull like a steel wire. By experiment it has been shown that in the case of water this cohesion or tensile strength is very great, for it requires over 200, and probably over 300, atmospheres to rupture a column of water.

The cohesion theory of the ascent of sap asserts that the water threads in the water channels of the wood are adhering to the wet walls of these vessels (and of the tracheids) and that these water threads can transmit any pull exerted on them at the top.

Any cross-walls in the wood channel do not interfere with the practical continuity of the water columns since these walls are fully soaked with water. As has already been explained transpira tion losses take place from the cells abutting on the intercellular spaces in the leaf, with a consequent increase in their suction pressure. As a result they take water from neighbouring cells and those with rising suction pressure will take water from the tracheids found in the fine vascular bundles (veins) of the leaf. The water columns in these dead tracheids are continuous with those in the rest of the wood channels in the plant, and the result of the withdrawal of water by the leaf cells is that these columns fall into a state of tension which is transmitted through out the plant and thus down to the roots. The pull of the mesophyll cells in the leaf is thus felt by the cortical cells of the absorbing root, and water is drawn from them into the wood and their suc tion pressure goes up. This leads to water being taken from cells farther and farther towards the outside of the root, and so to loss of water from the root-hair which takes water from the soil ; thus the chain of cohering particles extending from the soil to the leaf is complete.

Difficulties in the Cohesion Theory.—The cohesion theory cer tainly provides the most satisfactory explanation of the rise of the transpiration stream in tall trees. There are one or two dif ficulties, however, presented by the theory, one of which is in respect of air bubbles in the woody tracts. If sections of the wood are cut with great precaution a certain number of the wood channels are always found to contain air bubbles and therefore to be out of action. If the number of these air-containing chan nels went on increasing from year to year the proportion of blocked channels might be very high, though the new ring of wood laid down each year would start without air bubbles. It is suggested by Dixon that root pressure which develops in the spring would compress the air bubbles, by pushing the columns from below, and so cause their solution. It is doubtful, how ever, if root pressures occur of the magnitude necessary to bring about the solution of the bubbles at the top of a tall Sequoia tree. On the other hand the bubbles found in sections of the wood may be artefacts, i.e., produced in the cutting of the sec tions. From the observations of H. R. Bode it seems doubtful if bubbles exist at all in the water-conducting tracks of herbaceous plants under normal conditions, and the case may be the same in woody plants, though the bubbles seem to develop very easily; this question of air bubbles in the wood requires further investiga tion. Another difficulty lies in the behaviour of dead leaves. The cohesive theory it must be remembered is a purely physical theory. All it requires is one membrane evaporating into dry air, another membrane in contact with a water supply, and the two connected by a continuous water thread under such conditions that it can support a tensile stress. Accordingly if we kill a leaf with chloroform vapour while still attached to the plant it should continue to draw up water as its cell walls are still directly con tinuous with the water supplies of the stem. One would expect the leaf cells to collapse owing to the loss of osmotic pressure but not to dry up; we find, however, that such a dead leaf soon dries. This suggests that though the cohesion theory rightly envisages the main factors in the rise of water there are other subsidiary factors which play a part.