•78 ORGANOGRAPHY. 



terials necessary for the development of new structures, and then 

 the bark may be separated from the wood beneath, but such 

 separation can only be effected by the rupture of the cells of 

 which it is composed. 



3. Medullary Rays. — We have already seen that the stem at 

 its first development consists entirely of parenchyma, but that 

 in a short time fibro-vascular bundles are developed in this paren- 

 chyma, by which it becomes separated into two portions — an in- 

 ternal or pith, and an external or bark ; the separation however not 

 being complete, but the two being connected by tissue of the same 

 nature as themselves, to which the name medullary rays has been 

 applied. As new layers of wood are formed in successive years, 

 new additions are made to the ends of the medullary rays, by 

 which means, however large the space between the pith and the 



bark ultimately becomes, the 

 j^ig. Ibo. ^^,Q g^j^g always kept in con- 



nexion by their means. Be- 

 sides the medullary rays which 

 thus extend throughout the en- 

 tire thickness of the wood, 

 others are also commonly deve- 

 loped between them in each 

 succeeding year, which extend 

 from the zones of those years 

 respectively to the bark. These 

 are called secondary medidlary 



fij^.ies. Transverse section of a portion of rfl^'S. In tho Cork-oak both 

 the stem of the Cork-oak, four years u-i i^ ^^ • 



old. w. Pith. 1, Medullary ray of the ^"^^^ "^^7 ^^ ^^^^ ^^^n m a 



first year's growth. 2, 3, 4. Medullary transverse Section {jiq. 165, 



rays of successive years, pc. Liber i o q an T>ip inprlnllflrvvflvti 

 and raesophlceum. s. Corky layers. i-, ^, ^, "t)- ±ne meaullary layS 

 are composed of flattened six- 

 sided cells, which are placed one above the other in one or more 

 rows, like the bricks in a wall {Jig. 163, B, «, z ; and ^^. 68), 

 hence the tissue which they form is termed mzcriform farenckyma. 

 It is a variety of tahidar parenchyma as already noticed (p. 37). 

 The tissue formed by the medullary rays is rarely continuous 

 from one end of the stem to the other, but the rays are generally 

 more or less interrupted by the passage between them of the 

 fibro-vascular bundles, so that they split up vertically into a 

 number of distinct portions {figs. 166 and 167). This arrange- 

 ment may be observed by examining the surface of a stem from 

 which the bark has been removed {fig. 166), or still better by 

 making thin sections of the wood perpendicular to the rays, that 

 is tangential to the circumference of the stem {fig. 167). In 

 some stems, such as those of Aristolochia, the medullary rays are 

 very conspicuous, forming large plates between the wedges of 

 wood. In other plants, such as the Yew and Birch, they are 

 comparatively small. The medullary rays constitute the silver 



