VEGETABLE SAP. 117 



It has been observed above, that after an uncertain 

 period portions of the trunk cease to partake of the 

 diffusive current of the sap, and in those effete 

 members it becomes concreted. In this state it is very 

 visible in fir timber, reposing in knots and flaws, and 

 oozing out of the pores, when exposed to heat. When 

 chips of wood are submitted to maceration in a proper 

 menstruum, the secreted sap is first dissolved, next 

 the cellular partitions, and at last the woody fibres are 

 decomposed *. 



Although the natural motion of the sap be from 

 the roots to the extremities of the branches, there is 

 no doubt of it being occasionally transfusible in all 

 directions. Wherever there is depletion, thither will 

 the current be drawn; whether that be upward, 

 laterally, or downward. Whatever the position of 

 the branches may be, we see the sap courses along 

 them to reach every extremity. 



With respect to the old popular belief that the 

 whole body of sap regularly and annually retires to 

 the roots in winter, there to remain till the genial 

 warmth of spring again prompts it aloft, we may ob- 

 serve that the doctrine rests on no very solid founda- 

 tion. The vessels of the roots are never at that * 

 season so destitute of sap, nor have they capacity 



* In the case of pine or fir timber it is observable, that where the 

 cellular structure is most dense, as on the exterior sides of the 

 concentric layers, these portions are more durable than the inter- 

 mediate spaces. The medullary rays of these trees, though numerous, 

 are extremely thin. 



