110 COMPENDIUM OF GENERAL BOTANY. 



chyma. The water-reservoir of the root-system extends its influence 

 up to the medullary rays at the levels A. It is only necessary that 

 endosmosis (suction) should act from cell to cell through the paren- 

 chyma until the medullary -ray system B is reached ; here the water 

 is forced into the vessel until it rises to C. From there on endos- 

 mosis again acts within the parenchyma as far as the medullary 

 ray in D. The periodic oscillations in the bleeding-pressure 

 observed by various investigators are worthy of note. We may 

 assume also that in reference to bleeding a minimum, optimum, 

 and maximum temperature has some influence on this process. 1 



From Schwendener's communication on the ascent of cell-sap 

 the following important statements are selected. , 1. Every local 

 suction or pressure continues to act only in those particles of wood 

 in which there are connecting water- threads or columns. 2. Break- 

 ing of the water-columns or threads by air-bubbles produces a high 

 degree of immobility within the vessels. Air-bubbles in an iso- 

 lated vascular tube act differently from those in a tracheal system. 

 The Jamin's chain already referred to is formed in the vascular 

 tube. During the summer the bleeding-pressure in the stem of a 

 tree will allow only the escape of sap without any air-bubbles, 

 even when vessels and tracheids (libriform) are richly supplied with 

 air. This w r ater (without air-bubbles) comes from the tracheids, 

 and not from the vessels. The resistance in the Jamin-chain within 

 the vessels is too great to allow bleeding-pressure to set the entire 

 chain in motion. However, the mass of water can move onward 

 through the pores of the cell- walls by the completion of the water- 

 column through laterally uniting water- columns and threads in the 

 system of tracheids, while the enclosed air-bubbles remain station- 

 ary. As a rule, the air-bubbles are in the middle of the tracheids. 



The ready displacement of water within woody tissues suffi- 

 ciently supplied with water depends upon the presence of continuous 

 water-threads. Placing a drop of water upon a cross- section of 

 green wood several metres in length at once causes the escape of a 

 drop from the other end (" HAKTIG'S experiment "). 



A few words shall be added concerning the effect of rarefied 

 air, in other words concerning the physical process of suction (en- 

 dosmosis) within the wood. 



See PFEFFER'S Pflanzen physiologic. 



