ORGANS OF KTTRITION. 71 



in certain plants, as the large Umbelliferse, -^e find spiral vessels 

 in it. The latter howeyer are probably only detached portions 

 of the medullary sheath, separated in consequence of the great 

 horizontal distension to which such stems are liable from the 

 rapidity of their growth. 



2. The Wood. — This is situated between the pith on its inside 

 ; and the bark on its outer (fig. 155, r), and it is separated into 

 j wedge-shaped bundles by the passage through it of the medul- 

 lar)- rays. We have seen that in the first year's growth of an 

 exogenous stem the wood is deposited in the form of an inter- 

 rupted zone immediately surrounding the pith {fig. 160). That 

 portion of the zone which is first developed consists chiefly of 

 spiral vessels {figs. 160, i^; 161, t\ and 163, d), which form a 

 thin sheath, to which the name of medullary sheath is commonly 

 applied. This sheath does not however completely invest the 

 pith, as its name would lead us to believe, but it is interrupted 

 at certain points by the passage through it of the medullar}- 

 rays {figs. 155, h, 160, r). This is the only part of an exogenous 

 stem in which spiral vessels normally occur. 



On the outside of the medullary sheath, the zone of wood 

 firming the first year's gro^-th {fig. 163, 1), consists of woody 

 tissue (c), among which is distributed, more or less abun- 

 dantly, some vessels {b), chiefly of the kind called pitted in 

 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 

 cells of the cambium layer which are placed on the outside of 

 the first zone. This second zone {fig. 163, 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. 163, 3), and the 

 same is the case with each succeeding annual zone as long as the 

 plant continues to live. It is in consequence of each succeeding 

 layer of wood being thus deposited on the outside of those of the 

 previous years, that such stems are called exogenous. In the 

 stems of Gr}-mnospermous Plants, as those of the Fir, Yew, Cy- 

 press, the annual zones, which are well marked {fig. 164), consist 

 chiefly of disc-bearing woody tissue, with occasionally a few 

 pitted vessels intermixed. 



The pitted vessels, which we have seen form a portion of each 

 annual layer of the wood, are so large in the Oak, Ash. &c., that 

 they may readily be seen by the naked eye upon making a trans- 

 verse section of such trees ; and in all cases, upon examining 

 under the microscope a transverse slice of any common exogenous 

 stem, the pitted vessels may be at once distinguished from the 

 wood-cells by the larger size of their openings {fig. 161, v). In 



