MOVEMENT OF MATERIALS IN THE PLANT 



155 



Water loss through nectaries is still less significant in this connection. Sap pressure, 

 by which the water solution of the vessels is sometimes under pressure instead of 

 under tension (it is under pressure only when the transpiration rate is very low and 

 the soil water supply is plentiful), appears to be due to a sort of gland action (some- 

 what like that of hydathodes on leaf margins) in the tissues of the roots and other parts, 

 resulting in the active forcing of solution from the cortex into the xylem vessels; the 

 water thus forced into the xylem is derived from the surrounding tissue, and ultimately 

 from the soil. Bleeding, as of cut grape shoots in early spring, is partly or wholly due 

 to sap pressure. Sap pressure does not occur when the transpiration stream is rapid; at 

 such times the solution in the xylem vessels is under tension; therefore this phenom- 

 enon cannot generally be an important part of the cause of the rise of sap in stems. 

 This rise is directly due to the removal of water from the xylem above, to the tensile or 

 stretching strain transmitted through the water of walls, protoplasm, vacuoles, etc., 

 in all directions, and to the inward flow from the soil adjacent to the root surfaces. 

 The molecular physics of sap pressure, gland secretion, etc., is not yet understood. 



The rate of transpiration nearly controls the rate of water absorption in ordinary 

 plants with a plentiful water supply. When plants are well supplied with water at the 

 absorbing surfaces of the roots, the rate at which water is evaporated from leaves and 

 stems is dependent on several conditions, which may be grouped as internal and 

 external. Among the internal conditions are: The structure of the plant, the kind 

 of epidermis, the distribution, size, and open or closed condition of stomata, the 

 degree of water saturation of the tissues, the power of the foliage to absorb solar radia- 

 tion, the rate of water movement from roots to transpiring surfaces, etc. Many 

 (but not nearly all) stomata open and close according to conditions. Such stomata 

 usually open when the light intensity increases about dawn, and close more or less 

 completely with the diminution of light intensity in the evening. They also usually 

 close when wilting approaches. Generally the guard cells are more turgid when the 

 pores are open. Stomatal movement is due to changes in the turgor relations (ten- 

 sions) between the guard cells and the other epidermal cells. 



External conditions influencing the transpiration rate, when the root surfaces are 

 well supplied with water, are the evaporating power of the air (air temperature, air 

 humidity, air movement) and the intensity of absorbed sunlight. A large part 

 of the radiant energy absorbed may be converted into the latent heat of water vapor in 

 this way, without considerable change in the temperature of the foliage. When the 

 supply of water to the absorbing roots is not adequate, the rate of this supply greatly 

 influences the transpiration rate by limiting the rate of absorption by the roots. 

 Plants usually transpire more than they absorb during the day and absorb more than 

 they transpire during the night. 



There are usually several hundred stomata per square millimeter of leaf surface, 

 the stomata being frequently more numerous on one leaf surface than on the opposite 

 one. For plants of the same kind, all with the same environment and all having 

 been grown under the same condition complex, the amount of water lost per day 

 is about proportional to the extent of the leaf surface. According to Wollny, a maize 

 plant transpired nearly 13 liters of water during its entire growing season. A pea 

 plant correspondingly gave off nearly 4.5 liters. The transpiration rate is very 

 important in determining the rate at which dissolved salts, as well as water, enter the 

 roots from the soil, also in determining the rate at which dissolved salts are carried 

 to the leaves. The dissolved material is left in the leaves when the water evapo- 

 rates, and old leaves generally have a large salt content. 



