$2 THE WOOD OR XYLEM. 
in the form of an interrupted ring immediately surrounding the 
pith (fig. 182, t). That portion of the ring which is first developed 
consists, as we have also seen, chiefly of spiral vessels ( figs. 182, t, 
183, t, and 185, B, d), which form the medullary sheath. 
On the outside of the medullary sheath, the rg of wood 
forming the first year’s growth (jig. 185, B, 1) consists of woody 
tissue, c, among which are distributed, more or less abundantly, 
some vessels, b, chiefly of the kind called pitted in perennial 
plants ; although in herbaceous plants we have also annular and 
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Fig. 185. Diagram showing the structure of an Exogenous stem three years 
old. A. Transverse section. B. Vertical section. The figures 1, 2, 3, 
refer to the years of growth of the wood, and the letters mark similar 
parts in both sections. «a, a. Medulla or pith. d. Spiral vessels. 8, , 6. 
Pitted vessels. ¢, c,c. Wood-cells. e. Cambium layer. 7. Inner layer of 
bark or liber (phloém). g. Middle layer of bark. h. Outer layer of bark. 
i, i. Medullary rays. After Carpenter. 
other vessels. When the stem lasts more than one year a second 
ring of wood is formed, as we have seen, from the cells of the 
cambium layer which are placed on the outside of the first ring. 
This second ring (fig. 185, 2) resembles in every respect that of 
the first year, except that no medullary sheath is formed ; it 
consists therefore entirely of woody tissue and pitted vessels, 
c, b. In the third year of growth another zone of wood is 
produced precisely resembling the second (fig. 185, 3), and the 
same is the case with each succeeding annual ring as long as the 
