THE VEGETATIVE ORGANS OF PLANTS. 



279 



R 



as have been described. To answer the purpose of air 

 and sap-channels, the wood-cells which constitute the con- 

 centric rings of the old wood are constructed in a special 

 manner, being provided laterally with visible pores, through 

 which the contents of one cell may pass directly into those 

 of its neighbors. Fig, 

 53, _Z?, represents a por- 

 tion of an isolated wood- 

 cell of the Scotch Fir, 

 (Firms sylvestris^) mag- 

 nified 200 diameters. 

 Upon it are seen nearly 

 circular disks, #, y, the 

 structure of which, while 

 the cell is young, is 

 shown by a section 

 through them length- 

 wise. A exhibits such 

 a section through the 

 thickened walls of two 

 contiguous and adhering 

 cells. c, in both A and 

 J?, shows a cavity be- 

 tween the two primary 

 cell-walls; y is the nar- 

 row part of the chan- 

 nel, that remains while 

 the membrane thickens 

 around it. This is seen 

 in J?, y, as a pore or 

 opening in the cell. In 

 A it appears closed because the section passes a little to 

 one side of the pore. 



In the next figure, (54,) representing a transverse sec- 

 tion of the spring wood of the same tree magnified 300 

 diameters, the structure and the gradual formation of 



