ORGANS OF NUTRITION. 77 



in old stems. In tlie wood of some plants, as the Broom-rapes, 

 &c., no medullary sheath is formed. 



On the outside of the medullary sheath, the zone of -svood 



forming the first yciir's growth {fiy. 171, 1) consists entirely 



of woody tissue (c), among Avhich is distributed, more or less 



abundantly, some vessels (6), chiefly of the kind called pitted in 



Fig. 171. 



mm 



C <?/ ? /t. 



Fifj. 171. Diagram showing the struotui-e of a Dicotyledonons stem three 

 years old. A. Horizontal section. B. Vertical section. The figures 1,2,3, 

 refer to the years of growth, and the letters mark similar parts in both 

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

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

 (etidophlceum). g. Middle layer of bark (mesophheum). h. Outer layer 

 oi ha.i\ {epiphlceum). ?, t. Medullary rays. After Larpenter. 



perennial plants ; although in herbaceous plants we have also 

 annular and other vessels. When the stem lasts more than 

 one year a second zone of wood is formed, as we have seen, from 

 the cambium cells placed on the outside of the first zone. This 

 second zone {fig. 171, 2) resembles in every respect that of the 

 first year, except that no medullary sheath is formed, it consists 

 therefore entirely of Avoody tissue and pitted vessels (c, b). In the 

 third year of growth another zone of wood is produced precisely 

 resembling the second {fig. 171, 3), as is the case also with each 

 succeeding annual zone as long as the plant continues to live. 

 It is in consequence of each succeeding layer of wood being de- 

 posited on the outside of those of the previous years, that such 



