94 HIGHWAYS AND BYWAYS. 



Some of these questions may be answered by study 

 and experiment, while others the most scientific research 

 has failed to solve. The chemist can make the analysis, 

 but the synthesis which the tree has made surpasses liis 

 ability to perform. 



The most interesting problem in connection with this 

 study, and one perhaps the most diversely explained, is 

 the manner in which the fluids circulate or move from 

 one part of the tree to another ; but whether by expan- 

 sion, contraction, capillary attraction, endosmose, osmose 

 or permeation, experiment and observation teach that 

 most of the crude sap taken up from the earth by the 

 roots, after depositing some of its earthy matter in the 

 cells to thicken their walls, and taking in return gran- 

 ules of chlorophyl, is carried by the pleurenchyma, 

 parenchyma and duct cells to the extremities of the stem 

 or branches, there to nourish buds, leaves, flowers and 

 fruit, while the food, principally carbon, taken from the 

 air by the leaves, together with portions of vitalized sap, 

 is carried downward and deposited in the cambium 

 layer to form the new outer rim of wood and the inner 

 rim of bark ; thus showing, if there is not a complete 

 circulation of fluid, there is at least an upward flow 

 through one set of vessels, and a downward flow through 

 other channels. In connection with the respiration and 

 absorption of carbon by the leaves, how far they act as 

 capillaries in changing the fluids from one set of cells to 

 another is yet unknown, but there is little doubt that 

 the principal work of the foliage is to build up the wood, 

 while the root sap nourishes the foliage. 



