Exchange and Movements of Gases and Liquids. 5 7 



51. Bleeding. — If the action of the osmometers in § 44 

 is followed, it will be found that after the column has risen to a 

 certain height in the vertical tube the column remains sta- 

 tionary for a short time, then begins to fall. This is due to the 

 fact that the pressure forces some of the liquid taken into the 

 cavity of the dialyzer out through its walls. A similar occurrence 

 is to be found in plants. W'sXtx is absorbed by the concen- 

 trated cell sap in quantity, and a pressure set up in the cells is 

 sufficient to drive some of it out again through the walls. 

 As a consequence, while the root-hairs are absorbing soil solu- 

 tions in great volume from the soil, a small amount of liquid 

 is constantly being forced through their outer walls, carrying 

 substances capable in some instances of corroding rocks. 

 The solutions taken into the root-hairs are at the same time 

 withdrawn into the parenchymatous cells immediately under- 

 neath the epidermis, partly by this exudation pressure, but chiefly 

 by the osmotic attraction of the substances in these cells, 

 which also set up such pressure that some of the sap is forced 

 through their walls. As a result of this exudation, water is 

 forced into empty spaces outside the cells, and into the cavities 

 of the dead vessels and tubes in the fibrovascular tissues in 

 the roots. This action is not confined to the roots, however. 

 Wherever active thin-walled cells are supplied with water freely, 

 such exudation will ensue. If a stem or any organ in which 

 exudation is taking place is cut across, the sap in the spaces 

 and vessels will flow out, giving the appearance of " bleeding." 

 This phenomenon was first observed on the stumps of excised 

 shoots, and was supposed to be due to a pressure set up by the 

 roots, and is termed " root- pressure,'' even in many modern 

 text-books. As a matter of fact, this exudation may take place 

 in almost any part of the plant, and if a gauge is attached to 

 the end of an excised organ, the amount of the pressure may 

 be measured. It will be most convenient to take such obser- 

 vations on a stump of a stem cut down nearly to the roots, 



