Movement of Water in Plants (With Experiments)!
Ringing Experiments:
Most land plants obtain the necessary water for normal growth and development from the soil. By far, the largest proportion of absorbed water is lost as vapour in the process of transpiration from the aerial parts.
Much smaller quantities are utilised for growth and for various other metabolic processes of plants. Water, therefore, must move continuously through the intervening tissues and organs from the absorbing regions of roots to the tissues in which it is utilised or from which, it escapes as vapour, the leaves.
In small herbs and shrubs, the distance to be traversed by water on its onward journey from the root tips to the leaves, is usually not more than a few feet. But even in such plants, they may have such deep-seated root system, that the ascending water has to cover distances as great as 6-10 metres (m) before even reaching the surface of the soil.
It is in tall trees, however, in which the most striking illustrations of upward conduction of water occur. The tallest trees in record are a species of redwood in U.S.A., which attain immense heights of 90-120 m.
The heights of our own eucalyptus and firs range between 60—90 m. Since in all trees, the root systems also penetrate at least a few feet into the ground, the actual vertical distance through which the absorbed water must be conducted in order to reach the topmost leaves, may easily be 120 m or even more!
A simple calculation reveals that the force required to lift water to the top of tallest trees must be enormous indeed. For example, in case of a 90 m tree, a pressure of 10 atm. is just sufficient to support the column of water and according to all available experimental evidence, another 10 atm. is needed to overcome resistance due to friction.
So altogether a minimum of 20 atm. is necessary if the water is actually moving up through the xylem vessels. The mechanism by which this feat is accomplished against the force of gravity has been the subject of much speculation and it has been one of the most fascinating problems of plant physiology which has intrigued generations of investigators, not to speak of laymen. This has been termed, ascent of sap.
We know that water enters the land plant entirely through the root hairs and after crossing the cells of the cortex, the peculiarly thick-walled endodermis and pericycle, finally reach the xylem .vessels or tracheids of the roots.
At this point, its upward movement begins. We know that xylem vessels form a continuous conducting system, just like a pipeline, from the root tips through the main tissues of the roots and through the stems, the petioles of the leaves and ultimately through veins and veinlets, ending up in the surrounding mesophyll tissue of the leaves.
In the mesophyll cells, water moves onward from cell to cell and eventually the mesophyll cell walls, losing most of it by evaporation into the intercellular spaces, from where it escapes into the external air through stomata.
It was generally agreed, that xylem vessels are the main pathway of the upward conduction of water. A cell-to-cell lateral movement of water in a radial direction undoubtedly occurs along the vascular rays in the stems of most species of plants.
That xylem is the water- conducting channel in plants has been recognised as early as 1671 when Malpighi did his famous ‘ringing’ experiments. For over a hundred years now it has been definitely recognised that water is carried through the xylem, and that the other tissues, like pith, cortex, cambium, phloem are not directly concerned in the mechanism of water conduction in plants.
Among the reasons for this recognition of primary functions of xylem are the following:
(1) The anatomy of xylem obviously and clearly fits it for conducting purposes and it is also true that among the vascular tissues, xylem vessels have the right cross-sectional area to allow the upward transport of the large amounts of water required by plants;
(2) In the classical ringing experiments, it is observed that the removal of a ring of tissue external to xylem from the stem does not interfere significantly with upward movement of water to the organs situated above the ring whereas the removal of a cylinder of xylem from the stem certainly seriously disturbs or even completely stops conduction of water. The net result in many cases is almost complete wilting of leaves attached to the stem above the ring;
(3) Upward movement of water continues for some time even in shoots cut from plant, when placed in water. The rate of water movement (that is the length of the water column which will move past a given point) through xylem vessels varies greatly from almost imperceptibly slow, to speeds as high as 75 cm per min.
Ascent of Sap:
The mechanism of water moving vertically in plants against the force of gravity is still a classical problem of plant physiology. Since diffusion is much too slow to account for the rates that commonly occur in plants, mass movement of water must be envisaged; the whole column must move simultaneously instead of molecular movement.
A number of different theories of the mechanism by which ascent of water are brought about in plants has been suggested and the present state of our knowledge justifies a discussion of only three possible mechanisms:
(1) That the ascent of water is caused by the activity of the living cells, particularly of the stem—vital theories;
(2) That upward movement occurs as a result of root pressure;
(3) That ascent of water has its explanations in a set of purely physical principles, supplemented by cohesion of water theory, proposed by Dixon.
Vital Theories:
As living cells are more or less in intimate contact with the dead xylem elements through which upward conduction of water occurs; suggestions have often been made that the motive power of ascent of sap is provided by the vital activity of living cells. Our own J. C. Bose was the most ardent advocate of the vital theory of ascent of sap.
Bose’s idea was essentially an elaboration of the theory proposed by Godlewski in 1884. According to Godlewski, ascent of water resulted from periodic changes in the osmotic pressure of the living cells—wood-ray cells in the xylem.
It visualised a repetitive process of alternate increase and decrease in the osmotic pressure of the wood-ray cells, resulting in driving water out and into the xylem vessel. Thus the movement of water was supposed to be due to an alternate contraction and expansion of wood-ray cells, which also supplied necessary energy for the mechanism, perhaps energy released in cellular respiration.
Bose elaborated the idea still further. He claimed to have found perceptible galvanometric deflection of needle when an electric probe was very delicately pushed through stem tissue of Desmodium gyrans. The deflection of the needle was most noticeable when the probe reached the innermost layer of cortex, i.e., the row of cells just above the endodermis.
From this Bose concluded that the cells of the innermost layer of cortex have a sort of pulsating movements, just like the heart beats of animals—alternate contraction and expansion. According to Bose, these living cells with rhythmic pulsatory activities, acted as a sort of system of relay pumps.
As a cell contracted, it pumped up sap to the next higher cell, which (second cell) receiving the sap from the cell below (foot cell) expanded. The expanded cell (second cell) on contraction again pushed sap to the next higher cell (third cell), and so on.
Molisch reported confirmation of certain of Bose’s results, observing that moisture movement was closely linked to a characteristic rhythmic activity of the plant, which was amenable to control by drugs. Molisch, like Bose, concluded that the mechanism of the ascent of sap was mainly physiological rather than physical.
The concepts of Bose and Molisch have been destructively criticised by a number of investigators, including Smith, Benedict, MacDougal, and others.
Elaborate experiments carried out by these investigators, however, showed conclusively that water would continue to ascend for some time through the plant after all the living cells present in a woody stem were killed either by soaking the cut end into boiling water or by dipping it into a strong solution of picric acid.
However, it must be stated here that in many experiments the leaves at the top of such a treated stem, where all the living cells have been killed, sooner or later, in some cases after several days, showed definite signs of wilting.
The supporters of vital theories have accepted these phenomena of wilting of leaves as sure evidence that the living cells of the stem are essential for the conduction of water through it.
They argued that if the water-ascent was only through non-living xylem vessels in the stem, the leaves at the top of the treated twig would ever have remained fresh as long as the supply of water was there.
The exponents of vital theories, however, forget that the retardation of the conduction through the stem may have been due to much simpler causes than due to the direct killing of the living cells.
The plugging of the vessels by formation of toxic substances in the treated region of the stem or the transport of the same toxic substances into the leaves may well be the explanation of the wilting of the leaves in such experiments.
Even then, in all fairness, we admit the possibility that the living cells in the region of the xylem vessels may, in some way, contribute to the rise of water through the stem.
Root Pressure:
It has been claimed by some investigators that the positive hydrostatic pressure developed in the xylem vessels—root pressure—under conditions of excess absorption and low transpiration may be adequate to push the water to the tops of small herbs or shrubs.
While it is true that root pressure does, in some plants under certain conditions, account for the transport of water in the upward direction through plants, the process seems to be wholly insufficient to account for the rate at which water is known to travel through plants and also for the amount of water actually translocated.
In the second place, the magnitude of the root pressure developed is seldom adequate to raise water to the top of any, except perhaps trees of heights between 15-20m. at the most. For, the maximum observed root pressure rarely exceeds 2 atm. (i.e., a rise of 20 m.) and often much less. Furthermore, many of the tallest trees, particularly of the temperate regions, such as conifers, have no demonstrable root pressure.
The most conclusive evidence that the ascent of sap is not commonly due to root pressure is the fact that at time of rapid water movement in plants, “negative pressure”, (or tension) instead of positive root pressures, usually exists in the xylem vessels. This “negative force” has sometimes been called root tension.
This root tension is easily demonstrated in rapidly transpiring plants when a cut is made to remove the aerial parts and water is poured on the cut end of the stump. The water (sometimes as much as a gallon) is drawn back into the root immediately.
Physical Theories: Cohesion of Water Theory of Ascent of Sap:
In the application of physical principles for an explanation of the mechanism of ascent of sap in plants, atmospheric pressure naturally comes first to mind. But it is at once apparent that atmospheric pressure can only account for a rise of about 10 m. Suggestions were then put forward that water rose by imbibitional forces through the thick walls of the xylem vessels.
Physicists have shown that the forces of imbibition are very great, ranging from 100-1000 atm. and this at first sight would seem more than adequate for carrying water to any required height.
However, the rate of movement of water through imbibing colloids is extremely slow compared with known rates of water conduction in the xylem. Moreover, it was soon shown that water actually moves not through the cell walls, which would be required for an imbibitional transport, but through the lumen or cavities of the xylem vessels.
We have seen that negative pressure (or tension) generally exists in the xylem during rapid transpiration from leaves.
This can be shown by the use of a very sensitive instrument known as dendrograph which measures diameters of the tree trunk and such measurements definitely show, at times at least, a contraction of trunk-diameter during the day (caused by tension or pull due to transpiration) and expansion at night (tension less, due to stopping of transpiration).
Thus the ascent of sap is usually associated with a pull from above rather than a push from below (positive root pressure). Forces developing in aerial parts of the plants, especially in the leaves, cause the rise of water through the plant. It is easy to calculate mathematically whether this can be explained by a capillary pull in the vessels.
A vessel diameter of 0.1 mm would account for a rise of only 30 cm. But it must be understood clearly that it is not in the xylem vessels themselves that the main capillary pull occurs. Since this is clearly a surface tension phenomenon, the pull must occur at the water surface due to transpiration.
We know that water system of a plant is continuous from the lowest root hairs to the leaves at the topmost parts of a plant. The upper surface of the water column, therefore, is in the leaves, actually at the outer surface of mesophyll cells that are in contact with intercellular spaces that is in the microcapillaries of mesophyll cell walls. Since the microcapillaries are so fine, their diameters may be as small as or even less than 0.1µ (1µ = 
mm).
Taking the radius as 0.05 µ, a simple calculation reveals that as transpiration from the surface of microcapillaries on the cell walls takes place, they are capable of developing sufficient pull to support a water column about 300 m. high—nearly three times the height of the tallest trees!
The microcapillary pull, exerted on the mesophyll cell walls results in movement of water from the protoplasm into the cell walls and this in turn resulting in the movement of water from the vacuolar cell sap into the protoplasm lining the cell walls.
If the particular mesophyll cell is in direct contact with one of the branches of xylem cavities which spread like a network in the lamina of the leaf, water will be pulled out from the xylem vessel into the cell by the tension or suction developed in the microcapillaries of the cell wall. This results in the development of tension in the water column terminating in this particular xylem element.
Since the column of water moves as a whole this means transpirational pull is transmitted from the microcapillaries in the mesophyll cell walls to the protoplasm and vacuolar cell sap, to the adjacent cells and to the nearest vessel.
It is then transmitted down the vessels through the leaf blade, petiole, stem, and all the way to the roots. From the base of the vessel in the roots, the pull is transmitted through the adjacent cells of pericycle, endodermis, cortex to the epiblema cells and even to the medium surrounding the cells, i.e., external solution or soil particles.
Thus the transpiration pull, it seems, is responsible not only for the movement of water within the plant, but also of absorption of water from the external root medium, i.e., soil solution.
The following three stages in the process then may be distinguished:
(1) Transpiration from the surface of microcapillaries,
(2) Capillary rise of fine threads of water due to the force of adhesion between water and cell wall, and
(3) The whole column of water moves all together and resists breaking because of the force of cohesion between the water molecules—similar particles always have tendency to stick to each other.
Of course, these three stages occur simultaneously. This concept is known as Dixon’s transpiration-cohesion-tension theory of ascent of sap in plants. However, a column of water 90 m. high is subjected to a downward gravitational pull of 10 atm. all the time, tending to break the water column.
This powerful gravitational force is counteracted by an equal (10 atm.) upward surface tension pull due to transpiration. The most pertinent question that could be asked is that whether liquid such as water has sufficient tensile strength to resist the pull without rupture or whether forces of adhesion between the water and the vessel walls are sufficient to prevent a separation of the column of water from the vessel wall.
Physical chemists have experimentally shown that pure water molecules have a cohesive force of theoretically about 1000 atm. This is about 20 times more than the necessary cohesive force even for a 120 m. tall tree.
The tensile strength of water can be exhibited when the water is confined in very narrow tubes whose walls are rigid and incollapsible. Actually, values reported from plant cells for cohesive force of water range from 200-350 atm.; most striking values obtained are from the dead, thick- walled annulus cells of fern sporangia (300-350 atm.) at the time of violent bursting open of the sporangia for dispersal of spores.
But the water in the vessels is not pure. Besides solids, gases are present in solution. The reduction in pressure due to the capillary pull may, therefore, reduce the solubility of the gases until they separate from liquid and expand, causing rupture in the column of water.
Actually, vessels have, in fact, been found to become gas-filled. But since there are many columns of vessels side by side, it is not necessary for all of them to be continuous. The wet cell walls of vessels seem to be adequate in effectively preventing gas bubbles in one vessel from spreading into other units.
When the tension is relieved at night (due to stoppage of transpiration) or by rain, the gases in the vessels go back into solution and the columns of water may become continuous again.
Tensions in the vessels of up to 100 atm. have been reported. This points to the need of thick-walled xylem vessels in order to prevent a collapse of the vessels and stopping the flow of water.
Thus we see that in ascent of sap the water column is pulled upward en masse from the roots very much as a solid wire or rope may be pulled through a tube, only difference being that the water, since it is fluid, completely fills the cavity within the vessels. Ascent of sap is like lemonade sucked through a straw from a bottle on a hot summer day!
Although the aim of plant physiology is to explain all living processes in terms of known laws of physics and chemistry, the purely physical explanations of a living process seem, in most cases, incompatible with physiological evidence.
As a result it frequently becomes necessary to give a number of possible explanations of a particular phenomenon and to attempt to evaluate the merit of each one on the basis of accepted physico- chemical theories.
Thus, in the case of ascent of sap, it has been considered quite logical to discard (?) all other hypotheses proposed as explanations of the mechanism of ascent of sap from time to time except one, Dixon’s Cohesion of water theory, as it is not desirable in an elementary treatise such as this to discuss all of them critically.
The transpiration- cohesion-tension theory is generally accepted, perhaps with a pinch of salt, not because it looks good, but it reasonably seem to agree with some known facts.
During recent years, the supporters of classical cohesion theory had also to face drastic criticisms from several investigators in England and elsewhere. Handley, from his investigations on maple seedlings, came to the conclusion that living cells only are involved in the ascent of sap as his test plants showed pronounced wilting of leaves where the temperature of the seedlings were lowered below 2°C. There was a complete recovery of wilted leaves when the temperature was increased above the so-called critical 2°C.
As sufficiently low temperature had not been employed to freeze the water in the vessels, according to Handley, there should not have been any curtailment of water movement in the vessels. But actually there was pronounced wilting of the aerial parts.
Handley was strongly supported by Preston, who from a study of Handley’s findings, came to the same conclusion.
Handley’s observations, together with others discussed by Preston, have prompted the latter to conclude that only living xylem is involved in water transport, and both of them called for the complete discarding of the classical cohesion theory. Where are we now, then?
According to these investigators, a chain of living cells continuous from roots to leaves was involved in the ascent of sap in plants and that the vessels and the supporting elements are not active in the actual water conduction but merely serve as reservoirs, a view point strikingly reminiscent of the classical vitalistic school led by Bose.
Lundegardh also proposed his idea about water movement in trees. The most salient points among his suggestions were that only a very small proportion of total water in a stem is mobile and as vessels and larger tracheids in large woody dicots are air-filled, they can scarcely play any part in the conduction of water, the movement of water being confined almost exclusively in the narrower tracheids in which there may be continuous water columns from roots to leaves. The activity of the living cell in the wood was supposed to be a strong contributory factor.
Lundegardh’s suggestion that vessels play only a minor role in water conduction is not, however, corroborated by the experience of several other investigators who produced strong circumstantial evidence of the importance of vessels in water conduction in woody trees. Moreover Lundegardh’s suggestion that the narrower, medium-sized tracheids are the main channels of water transport in trees is certainly not keeping with marked higher rates of water transport in dicotyledonous trees as opposed to those in conifers.