ABSORPTION AND MOVEMENT OF WATER 133 



amounts and the kinds of organic matters will vary not 

 only with these two factors, but also with the condition of 

 the individual, both at the time, and during the foregoing- 

 season, when organic substances were being formed and 

 stored by the plant. 



Sap-pressure, which determines the rate of sap-flow and of 

 bleeding, varies at different times in the season and in the 

 day. The amount of water in the soil and of moisture in 

 the air will directly affect the physical conditions of sap- 

 pressure and of sap-flow. If the soil is dry, only small 

 amounts of water can be absorbed, the turgor-pressure in 

 the living cells of the root will be relatively low, compara- 

 tively little water will be pressed from these cells into the 

 wood-elements, and therefore the sap in the wood will in- 

 crease proportionally little in volume and pressure. If the 

 air is dry, more water will evaporate from the plant, the 

 ratio between the amounts absorbed and given off will be 

 lowered, and the volume and pressure of the sap in the wood 

 will be proportionally lowered. 



Other factors, acting directly upon the protoplasm and 

 only by this means affecting the physical conditions of sap- 

 pressure, cause the pressure and rate of flow to vary from 

 time to time. Whatever stimulates the protoplasm of those 

 cells in which food is stored in solid form to dissolve the 

 food, will tend, other things being equal, to raise the sap- 

 pressure by increasing the absorption of water and the 

 turgor-pressure. The increased secretion by the protoplasm 

 of diastatic or other enzyms, by means of which more in- 

 soluble solids will be converted into osmotically active solu- 

 ble substances, increases the absorbing power of the cell-sap 

 and proportionally increases its volume and pressure. Con- 

 versely, whatever influences depress the protoplasmic activi- 

 ties also tend to reduce the sap-pressure. 



Experiments by Wieler* on plants in pots, and there- 

 fore under controllable though somewhat artificial con- 

 ditions, confirm the observations made on plants in 



* Wieler, A. L. Das Bluten der Pflanzen. Cohn's Beitrage zur Biologic 

 der Pflanzen, Bd. VI., 1893. Gain, E. Action de Peau du sol sur la veg- 

 etation. Revue Generate de Botanique, t. VII., 1895. 



