110 BOTANY. 



except in the centre, which is traversed by a woody cylinder 

 (fibro-vascular bundle) of a lighter color. This is sometimes 

 circular in sections, sometimes horse-shoe shaped. Where the 

 stem branches, the bundle of the branch may be traced back 

 to where it joins that of the main stem. 



A thin cross-section of the stem shows, when magnified, three regions. 

 First, an outer row of cells, often absent in the older portions ; this is the 

 epidermis. Second, within the epidermis are several rows of cells similar 

 to the epidermal cells, but somewhat larger, and like them having dark- 

 brown walls. These merge gradually into larger cells, with thicker golden 

 brown walls (Fig. 67, /). The latter, if sufficiently magnified, show 

 distinct striation of the walls, which are often penetrated by deep narrow 

 depressions or " pits." This thick- walled tissue is called " stony tissue" 

 (schlerenchyma). All the cells contain numerous granules, which the 

 iodine test shows to be starch. All of this second region lying between the 

 epidermis and the fibro-vascular bundle is known as the ground tissue. 

 The third region (fibro-vascular) is, as we have seen without the micro- 

 scope, circular or horse-shoe shaped. It is sharply separated from the 

 ground tissue by a row of small cells, called the "bundle sheath." The 

 cross-section of the bundle of the leaf stalk resembles, almost exactly, that 

 of the stem ; and, as it is much easier to cut, it is to be preferred in study- 

 ing the arrangement of the tissues of the bundle (Fig. 67, G). Within the 

 bundle sheath (sh.) there are two well-marked regions, a central band (x) 

 of large empty cells, with somewhat angular outlines, and distinctly 

 separated walls ; and an outer portion (y) filling up the space between 

 these central cells and the bundle sheath. The central tissue (x) is called 

 the woody tissue (xylem) ; the outer, the bast (phloem). The latter is 

 composed of smaller cells of variable form, and with softer walls than the 

 wood cells. 



A longitudinal section of either the stem or leaf stalk shows that all the 

 cells are decidedly elongated, especially those of the fibro-vascular bundle. 

 The xylem (Fig. 68, (7, x) is made up principally of large empty cells, with 

 pointed ends, whose walls are marked with closely set, narrow, transverse 

 pits, giving them the appearance of little ladders, whence they are called 

 " scalariform," or ladder- shaped markings. These empty cells are known 

 as " tracheids," and tissue composed of such empty cells, "tracheary 

 tissue." Besides the tracheids, there are a few small cells with oblique 

 ends, and with some granular contents. 



The phloem is composed of cells similar to the latter, but there may 

 also be found, especially in the stem, other larger ones (Fig. 67, </), whose 



