Pressure and Flow of Sap in the Maple

The source of pressure for the bleeding of trees in spring seems to be localized in different portions of the plant in different species. 2. Trees may be roughly classified into two groups in this regard: (a) those in which the source is mainly in the trunk and branches. These, as for example the maple, bleed early in the season. (b) Those in which the source is in the root only. These bleed later in the spring, as for example the birch and grape. 3. Only the problem of the cause of pressure in the maple is considered in this paper. 4. There seems to be an undoubted relation between the occurrence of pressure in the maple and the fluctuations in temperature. 5. In connection with no other fluctuating factor of the environment can such relation be shown. 6. Pressure exists only when the temperature is rising. When it falls or remains constant, suction occurs.