Grafts et al. — 174 — Water in Plants 



Translocation of Solutes :— Work by Maskell and Mason (1929) and Clements 

 and Engard (1938) on inorganic solute movement, and by Stout and Hoagland 

 (1939), and Bennett and Snell (1939) using radioactive indicators shows that the 

 major primary movement and distribution of mineral nutrients in plants takes place in 

 the xylem in conjunction with the transpiration stream. Hoagland (1944) has em- 

 phasized that absorption of water and salts are largely independent, and the selective 

 uptake of nutrient ions by living cells during their upward passage through the xylem 

 further indicates their independent behavior. However, while they occur together to 

 make up the xylem sap there seems to be no reason for assuming an independence of 

 movement; once started on their way through the xylem tubes, water and salts move 

 along together, and, with the exception of those absorbed along the route they reach 

 their final destination in the leaves where the bulk of the water is lost by transpira- 

 tion. 



Much of the salt absorbed by roots is used in the building of leaves. However, 

 when these have accumulated sufficient to meet their needs for growth and the osmotic 

 retention of water, there is evidence (Mason and Maskell, 1931) that the surplus is 

 moved out via the phloem in conjunction with carbohydrates. 



Concerning food movement in plants, there is general agreement that gradients in 

 concentration are intimately involved (Mason and Maskell, 1928a, 19286; Curtis, 

 1935; LooMis, 1935, 1945; Engard, 1939a; Huber, Schmidt, and Jahnel, 1937). On 

 the mechanism of movement there are many theories ; these may be grouped into two 

 categories : 1 ) those postulating movement of solute molecules in, through, or upon 

 the surface of the sieve-tube protoplasm. By this mechanism solutes are pictured as 

 moving along gradients but independently of each other and of the solvent water; 2) 

 the mass-flow or convectional-flow theory that visualizes a flow of solution through 

 the phloem along positive gradients of hydrostatic pressure developed osmotically. The 

 first category includes several mechanisms including diffusion accelerated by proto- 

 plasmic streaming (Curtis, 1935), "activated diffusion" a term implying an ac- 

 celerated diffusion carried on by the protoplasm with the utilization of metabolic energy 

 (Mason and Maskell, 1937), and diffusion or some analogous process accounting 

 for rapid attainment of equilibrium by molecules adsorbed on the surface of the sieve- 

 tube protoplasm (Loomis, 1935, 1945; Clements, 1934a, 1940). All these call upon 

 the active protoplasm for the required energy and their advocates picture the function- 

 ing sieve tube as an active element, rich in protoplasm, and capable of expending 

 metabolic energy at a high rate. 



The second category includes only one mechanism but presents two views of the 

 sieve-tube function. That of Munch (1930), introducing the symplast concept, 

 visualizes the interconnected protoplasts of the plant as constituting a continuous sys- 

 tem, relatively impermeable on its outer surface but highly permeable throughout its 

 mass. The sieve tubes are pictured as making up a specialized mechanism of the 

 symplast where water, absorbed osmotically, brings about high turgor in the regions 

 of synthesis. This turgor promotes a rapid mass-flow of solution through tubular con- 

 nections that traverse the sieve plates to regions of utilization where assimilation, res- 

 piration, and condensation for storage reduce the osmotic activity of the solutes allow- 

 ing the water to return to the xylem. Crafts (1938, 1939a), on the basis of anatomical 

 studies, contends that the mature, functioning sieve tubes become permeable, allow- 

 ing a ready passage of solutes in solution from cell to cell. Leakage to the outside is 

 prevented by a limiting layer of living cells around each sieve-tube strand. Movement 

 from mesophyll to sieve tube is by diffusion, accelerated by protoplasmic streaming via 

 the symplast. Removal of assimilates from the sieve tubes is pictured as an active 

 absorption by living parenchyma cells of the phloem followed by symplastic move- 

 ment to points of utilization or storage. The mass-flow mechanism involves a recircula- 

 tion of water; Munch calculates that around 5 per cent of the total water used by 

 the plant is involved in this recirculation. He performed experiments wherein this 

 water as it flowed from the cambium of trees was collected and measured. 



The protoplasmic theories are supported by evidence that under certain conditions 

 solutes appear to move independently in the phloem, that the sieve tubes are rich in 

 proteinaceous materials indicating a high protoplasmic content, and that radioactive 

 phosphorus absorbed into the phloem of leaves moves both upward and downward in 

 the stem whereas mass flow should be predominantly in one or the other direction. The 

 greatest weakness of these theories is that no exact physical mechanism is known or 

 has been pictured that will account for rapid transport of the quantities of material 

 found to move either along the protoplasm or through the lumen independent of the 



