EXUDATION-PRESSURE : ROOT-PRESSURE i8; 



bordering on the wood-strands {w, 2, Fig. -72), the protoplasm 

 of the former becomes permeable, and a portion of the cell-sap 

 within them is forced into the cavities of the vessels and tracheids 

 with which the cells are in contact. 



The pressure thus set up by the turgid parenchymatous cells 

 of the cortex and the cells of the ground tissue within the 

 vascular cylinder of a root is termed root-pressure. 



Under this pressure the vessels and tracheids of the vascular 

 bundles become filled with water, and on cutting off the stem of 

 a tree in spring after the roots have begun their absorptive work 

 and before the buds have opened, the water is forced out of the 

 cut end of the stump still connected with the root in larger or 

 smaller quantities; such outflowing of water from plants which 

 have been cut is spoken of as 'bleeding.' The liquid forced out 

 of a 'bleeding' plant is not pure water, but a solution containing 

 small quantities of various substances, such as soluble carbo- 

 hydrates, acids, organic and inorganic salts, and proteids. In 

 the sugar maple the liquid contains over 3 per cent, of sugar 

 which in some parts of the world is profitably extracted from it. 



In the case of the vine, sycamore, birch and other trees, 

 'bleeding' may continue for several days, during which time 

 several pints of ' sap ' may be exuded. 



By attaching a suitable manometer or pressure-gauge to the 

 stump of a ' bleeding ' stem, the pressure with which the sap is 

 forced out can be measured : in the vine it frequently amounts 

 to more than one atmosphere, or sufficient to support a column 

 of mercury 760 mm. in height. 



The root-pressure of a stinging nettle was found to be sufficient 

 to balance a column of mercury 460 mm. in height, while that 

 of an ash tree was only able to support a column of 20 mm. of 

 mercury. 



The phenomena of root-pressure and 'bleeding' are best 

 observed in woody perennials, such as the vine, birch and 

 sycamore, in spring and early summer about the time when the 



