MOVEMENT OF MATERIALS IN THE PLANT 141 



a mercury manometer (Fig. 81). The mercury is forced up in the free arm of 

 the tube by the pressure of the exuding sap, attaining a height, in one of Hales' 

 experiments, of 103 cm. or about 1.5 atmospheres. Instead of removing a 

 branch, an incision may be made in the stem. Bleeding is characteristic of 

 many woody plants in the spring; this is called spring bleeding, since it occurs 

 only in the spring before the leaves expand. After the leaves expand an incision 

 in the stem or the removal of a branch usually fails to produce bleeding; water 

 is then being lost from the leaves by transpiration. Under these conditions 

 bleeding may be induced at the surface of the stump of a cut stem, the leafy 

 portion having been entirely removed. Bleeding may be demonstrated in 

 this way throughout the entire vegetative period, in both woody and herbaceous 

 plants, but the same plant may not show it at all times during its period. 



To measure the force with which sap is extruded, a mercury manometer 

 is connected to the cut stump of the plant. To measure the amount of liquid ex- 

 creted the manometer may be replaced by a glass tube connecting with a 

 graduate. The recording apparatus of Baranetskii serves the same purpose. 

 Here the liquid flows into a U-shaped tube, lifting a float in the free arm. The 

 float is fastened to one end of a silk thread that passes over a pulley, and a 

 pointer attached to the other end of the thread traces a curve on a smoked, rotat- 

 ing drum. In another apparatus constructed by Baranetskii, the excreted 

 liquid is caught in separate tubes, each tube remaining beneath the outlet tube 

 from the plant for a single hour. The tubes are arranged on the rim of a wooden 

 disk with vertical axis, and this is rotated, by clockwork, just far enough every 

 hour to place a fresh tube under the outlet. 



Exudation pressure, as indicated by the height of a mercury column, varies 

 in different plants, being less in herbaceous than in woody forms. Thus, in 

 Hofmeister's 1 experiments the height attained by the mercury column was 

 66 mm. with A triplex hortensis, and 461 mm. with Digitalis media. 



The amount of sap excreted by herbaceous plants greatly exceeds the total 

 volume of their roots. Much of the excreted liquid must therefore enter the 

 roots after the cut is made. A plant of Urtica wrens excreted 3025 cc. of sap, 

 and the total volume of its root system proved to be only 1350 cc. Similarly, 

 the root volume of a plant of Helianthus animus was only 3370 cc, and yet 

 this plant excreted from its cut stump 5830 cc. of liquid.'' 



There is a daily periodicity in the rate of bleeding 2 and this has no relation 

 to temperature. The time of occurrence of the maximum and of the minimum 

 rate of liquid excretion is not the same for different plants. Etiolated plants 

 exhibit no periodicity. Analyses of the sap extruded by bleeding stems are 



'Hofmeister, W., fJeber das Steigen des Saftes der Pflanzen. Flora, n. R. 16: [-12. [858. Idem, 

 [Jeber Spannung, Ausflussmenge und Ausflussgeschwindigkeit von Säften lebender Pflanzen. Ibid. n. R. 

 20: 07-108. 1862. 



' Baranetzky, J., Untersuchungen über die Periodicität des Blutens der Krautigen Pflanzen und deren 

 Ursachen. (Besonders abgedruckt aus den Abhandl. Naturf. Ges. Halle 13.') 63 p. Halle, 1873 



''In this connection see: Hofmeister, W., Ueber Spannung, Ausflussmenge und ausfluss- 

 geschwindigkeit von Säften lebender Pflanzen. Flora 45 : 97-108, 113-120, 13S-144. 145- 

 152, 170-175. 1862. The numbers given in the text are from this paper. — Ed. 



