NATURE AND SOURCES OF FOOD. 97 



to their vital action, undergoes a change of properties. The 

 water thus altered is called the crude or ascending sap. This 

 fluid, in the active periods of vegetation, is incessantly in 

 motion, and is unquestionably analogous to the blood of ani- 

 mals. But the motion of the sap in plants is a great deal more 

 complicated and altogether different from the circulation of the 

 blood in animals. The sap is not, like the blood, confined to a 

 separate system of vessels, for owing to the manner in which 

 the vascular and cellular tissues are interwoven with each 

 other, and the general permeability of all the organs, a general 

 transfusion of the sap from cell to cell takes place endosmoti- 

 cally in every direction. This is particularly the case at the 

 commencement of growth, as in germinating plantlets, or 

 developing leaf-buds, but as soon as woody fibre and vascular 

 tissue or ducts are formed, they take the most active part in 

 the upward conveyance of the sap for which they are well 

 adapted by their tubular and capillary character. 



The current of ascending sap flows through the vitally active 

 and forming cells of the alburnum or sap-wood, situated nearest 

 the bark, and not at all through the dead wood cells of the 

 duramen or heart-wood, situated in the interior of the stem. 

 It is this interposed stratum of sap which renders the bark and 

 wood so easily separable in the spring of the year. 



The sap in plants appears to be set in motion by the expan- 

 sion of the buds. The extremities of the branches are always 

 more herbaceous than the part of the branches immediately 

 below them, and therefore are the first to be affected by an 

 increase of temperature in early spring. So soon as the extre- 

 mities of the branches together with the buds begin to swell, 

 the cells of which they are composed attract the sap from the 

 tissues in their immediate neighborhood, which tissues are 



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