72 



OBGANOGEAPHY. 



the Coniferse, where but few if any pitted vessels occur, a trans- 

 verse section shows the orifices nearly all of equal size, with 



Fig. 163. 



d i i a 



c €/ j h 



Fig. 163. Diagram showing the structure of a Dicotyledonous stem three 

 years old. A. Horizontal section. B. Vertical section. Tlie figures 

 1, 2, 3, refer to the years of growth, and the letters mark similar pang 

 in both sections, a, a. Medulla or pith. d. Spiral vessels, h, b, b. Pitted 

 vessels, c, c, c. Wood-cells, e. Cambium layer. /. Inner layer of bark, 

 or liber (endnphlceum). g. Middle layer of bark {mesophlceum). h. Outer 

 layer of bark {epiphlaum). i, i. Medullary rays. After Carpenter. 



occasionally a pitted vessel intermixed. These pitted vessels 

 in ordinary trees are also commonly more abundant on the inner 

 part of each annual zone, the wood- cells forming a compact 

 layer on the outside (fig. 163). In such cases the limits of each 

 zone are acciirately defined. In those trees which have the 

 pitted vessels more or less diffused throughout the woody tissue, 

 as in the Lime, Maple, &c., the zones are by no means so evident 

 (Jiff. 161), and can then only be distingtiished by the smaller size 

 of the wood-cells on the outside of each layer, which appearance is 

 caused by their diminished growth towards the end of the season. 

 The distinction between the annual zones is always most evident 

 in trees growing in temperate and cold climates, where there is a 

 more or less lengthened winter in which no growth takes place, 



