OSMOSIS (Neo-Lat., from Gk. 410'1165, &)nOS, 11111Mision, from dptkey, Stlicin, to push). This phenomenon, which is discussed in its funda mental form and from the physical point of view under SOLUTION, plays an important part in plant physiology. The plant cell, consisting of a mass of protoplasm, surrounded by a somewhat tough lint flexible and elastic memlirane, the cell wall, amp inelosing a watery fluid, the cell sap. is an appropriate mechanism for an effective dis play of osmotic phenomena, which are fundamen t:id to three important functions: ( 1) turgor (q.v.). (21 absorption and transfer of water. (31 absorption and transfer of dissolved substo flees'. ( See ItSolIPT ION Ix PLANrs.) The genera I prin ciples set forth under Sold_ T ION 11101 be summar ized with respect to plants as (a ed substances diffuse to the limit, of the sol vent and exert an osmotic pressure comparable to gas pressure and main testing the same laws.
lb I The osmotic pressure substances in sulutiou together equals the sum of the pressures of each salute upon the given V0' 111114' of sole (mt. (c) osmotic pressure may be made evident by interposing a membrane between solutions of unequal pressure, the soltent freely passing, while the solutes are hindered or pre vented. (d) In plants water is the sole solvent. The plant cell is an osmotic apparatus. The cell wall is ordinarily permeable to both solvent and solutes, but the protoplasm is a semi-permeable membrane with respect to many solutes. Under normal conditions, when is avail able, it enters the cell and allows the osmotic pressure of the solute's to manifest itself in push ing the protoplasmic membrane against the cell wall until the elastic resistance of the wall bal ances the iwessurc. A cell with wall distended is said to be turgid. The presence of solutes in the Water within the semi-permeable membrane of protoplasm in the surface cells of the root de mands the entrance of water from the outside until an equilibrium is reached. But the attain
ment or maintenance of equilibrium is constantly prevented by the evaporation of water from sur faces exposed to the air; for when water evap orates from a cell the elastic resistance of the cell wall is no longer equal to the osmotic pres sure and water enters from a neighboring cell, and so on, corresponding action making possible the movement of water from more and more re mote cells until the turgor is equalized liy the en trance of water from without. (See CONDUC TION.) The absorption of solutes from water in contact with the plant is independent of the movement of water. If by removal from the cell sap, either by use or by transformation, the os motic partial pressure of any solute within the cell be diminished, other molecules of this solute may gain entrance. Provided the protoplasm be permeable to it, the entrance of any solute de pends on a disturbance of the equilibrium between the osmotic partial pressure of that substanee in side and outside the cell.
Since the osmotic pressure of solutions of acids and salts (electrolytes) is greatly increased by their elect rolytie dissociation ( see DISSOC1A iIoN), wIciett occurs in watery solutions, this must be an important factor in plant life. Nor mally all the solutions surrounding the plant are dilute watery solutions in which the extent of such dissociation is considerable. The solutions within the plant cell, too, contain solutes in a state of electrolytic dissociation. In considering the osmotic phenomena within the plant.therefore, allowance must be made for the increase of os motic pressure due to electrolytic dissociation.