ROOT-PBESSUBE 



89 



force it through the stem, so that an exudation after a time 

 can be noticed to take place from the cut surface which is 

 not immersed. A similar exudation can be caused to take 

 place from the hypha3 of fungi and from the tissues of 

 mosses. 



We must, however, be cautious not to attribute every 

 escape of water from a plant to this cause. When a tree 

 trunk is wounded or cut on a warm sunny day in winter, 

 there is frequently an exudation of water from the wound. 

 This is generally due to purely physical causes, being 

 brought about by the expansion of 

 the air which is contained in the 

 vessels of the wood. It can be 

 artificially produced at any time 

 in winter by warming a freshly 

 cut piece of wood ; and its cause in 

 this case can be seen to be physical 

 by the fact that as the wood cools 

 the water in contact with the cut 

 surface is again absorbed, owing to 

 the contraction of the air, which 

 was expanded by the warming. 



To measure the root-pressure 

 in a plant the apparatus shown in 

 tig. 62 may be used. It consists of 

 a T-piece of glass tubing (R), which 

 is fastened by indiarubber rings 

 (r) to the top of a cut stem, such 

 as that of Helianthus. To the side 

 arm of the tube a manometer (q), 



with a capillary bore, is attached by a tightly fitting cork 

 (ft), and the T-piece is filled with water from the upper end 

 (&'). Mercury is poured into the manometer till it stands 

 at a level a little below the cork k, and the aperture ~k' is 

 then tightly closed. As the root continues to take up water, 

 it forces it into the tube E, whence it overflows into the 

 proximal arm of the manometer, causing the mercury in 



