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the leaves are developed, what is called bleeding by gardeners 

 occurs, that is to say, that water will gradually and slowly ooze 

 out from the cut surfaces ; and closer investigation has shown 

 that this water exudes from the openings of the large vessels. 

 This may also be seen by cutting through several trees, as a 

 Maple or a Birch, when a flow of water of considerable strength 

 will take place, many pints of fluid being poured out from the 

 cut surface for some time. It may also be seen in all woody 

 shrubs growing vigorously and provided with a well-developed 

 root system. If we cut through the stem of a Sunflower or of a 

 Tobacco plant, a few centimetres above the ground, and prevent 

 the evaporation which would take place from the cut surface, 

 an outflow of fluid will begin after a time, which may continue 

 for several days. 



The boring or tapping of Maples for Sugar by means of holes 

 is effected in the spring-time, for the longer stagnation in the 

 cavities of the wood during winter gives the water the power 

 of absorbing sugar out of the closed living cells of the wood, 

 and out of the parenchyma surrounding them ; a result which 

 cannot be expected, or only in a lesser degree, in the case of the 

 rapid flow from the smaller root-stalks of quickly growing 

 plants. 



The presence of fluid in the vessels in the spring cannot 

 possibly be due to the leaves, because they are either not un- 

 folded at all, or, if they are, they are very small indeed, and, 

 hence it must be due to some force exerted by the roots them- 

 selves. This force, known as the root pressure, is occasioned 

 thus : the water is absorbed from the soil by the roots, and 

 forced by means of the hydrostatic pressure set up in the par- 

 enchyma cells into the cavities of the vessels of the roots by a 

 process of filtration, thence it is forced from the roots into the 

 stem, so that when the latter is cut it is poured out. The 

 amount of pressure exerted by this force of outflow is very con- 

 siderable and easily perceived, and the amount of fluid given 

 out very large ; it may be measured by attaching a glass tube 

 containing mercury to the cut surface, when we find that the 

 pressure is able to raise up the column of mercury for a con- 



