97 



Such " bleeding " may continue for several days, or even 

 weeks, and if the roots are kept warm and well supplied with 

 water the quantity of fluid exuded may be large, and often 

 considerably exceeds in volume the total volume of the root 

 system. 



That this so-called root-pressure, however, in itself does not 

 suffice to explain the ascent of the water current in high trees is 

 shown by the following facts. If the total quantity of water 

 given off by such bleeding stems is measured it is found to be 

 very much less than the amount actually required and given 

 off by the transpiring leaves. Again if an actively transpiring 

 plant in full leaf is cut, the stump, instead of exuding water, 

 will at first actively absorb it, and water placed on the cut section 

 will be taken up by the vessels and tracheids, thus showing 

 that, instead of a positive root-pressure existing in the latter, 

 the existing pressure is less than that of one atmosphere. It 

 has been ascertained also that when transpiration is actively 

 proceeding the water does not occur in continuous columns 

 in the conducting elements, but as short columns, alternating 

 with bubbles of air, and that the pressure in them generally 

 decreases towards the top of the tree. 



Now, if for any reason there exists a demand for a particular 

 substance in a certain cell, that substance will continue to 

 be attracted to the cell until the demand is satisfied. If. 

 for instance, owing to the presence of a concentrated solution 

 of a substance having a strong attraction for water, an osmotic 

 current into the cell is created, and further, if this water is driven 

 off in the form of vapour as fast as it enters the cell, the inward 

 current will continue to flow, since the needs of the cell in res- 

 pect of water remain unchanged. If, on the other hand, 

 the water is allowed to dilute the solution and to continually 

 carry off some of the same to neighbouring cells by exosmosis, 

 a condition of equilibrium will sooner or later be reached, 

 the concentration of the solution in all the cells being the 

 same. Glucose, or grape-sugar, is a soluble substance derived 

 from starch by the addition of a molecule of water, and if we 

 suppose that a dilute solution of glucose is flowing into a cell 

 by osmosis and that on reaching the cell the glucose becomes 

 converted into insoluble starch, which is deposited as starch 

 grains, the inward flow of glucose will continue, the cell's needs 

 in respect of glucose remaining unsatisfied. It is in this way 

 that large quantities of the so-called reserve food-materials 

 are accumulated and stored in certain cells until they are 

 required. Similarly, although currents of water with salts 



