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which it is exhaled in considerable quantities. The liquid 

 absorbed by the roots is not forced by their osmotic activity 

 to the leaves, but only into the vessel-walls, and thence it is 

 sucked up in obedience to transpiration. We have still to 

 enquire how the water gets from the vessels of the leaves, in 

 the vessel-walls of which it exists, to the actual cells which 

 exhale the watery vapour. These latter are not wood cells, but 

 parenchymatous cells containing chlorophyll grains for the 

 most part, and making up the great mass of the tissue or 

 mesophyll of the leaf. Water has then to get to these from the 

 vessels, and this is accomplished in a similar manner to the 

 original mode of absorption of the water by the roots. The 

 vessels contain in their walls then a considerable quantity of 

 water taken up from below ; this is absorbed by the parenchy- 

 matous cells adjoining them in virtue of their osmotic activity, 

 which in these cells, as in those of the root, is always greater 

 than their cubic content, i. e., if the cell-walls allowed of their 

 expanding they could take up more water ; this water taken up 

 from the vessel-wall is then passed on from cell to cell towards 

 the exterior by osmosis, and exhaled in the form of watery 

 vapour into the intercellular passages, and thence through the 

 stomata. 



Hitherto we have considered herbaceous plants ; but in trees, 

 all the vessels do not transmit fluid in their walls. The wood 

 in a Dicotyledonous stem which increases in thickness, at 

 length comes to consist of a central portion which is older, 

 darker coloured, and harder than the rest, called the duramen or 

 heart- wood ; and of an outer, younger, and lighter coloured por- 

 tion which is called the alburnum or sap-wood. Of these two 

 portions the alburnum is the only part which is active in this 

 process and fitted by its organization to transmit fluid, and 

 which is likewise in connection with the leaves of the year ; 

 the duramen, on the other hand, soon dries and becomes more 

 and more incapable of transmitting fluids, so that it takes no 

 part in this conduction of water. This is illustrated by the 

 vegetation of hollow forest trees, in which a sufficient layer 

 of young wood remains within the bark to carry up the 



