64 BOTANY. 



are somewhat oval in outline, with the pointed end directed toward the centre 

 of the stem. If we look at the section as a whole we see that there is a nar- 



Fig. 52. 

 Xylem portion of bundle. Cambium portion of bundle. Bast portion of bundle. 



Section of vascular bundle of sunflower stem. 



row continuous ring * of small cells situated at the same distance from the 

 centre of the stem as the middle part of the bundles, and that it divides the 

 bundles into two groups of cells. 



115. Woody portion of the bundle. In that portion of the bundle on the 

 inside of the ring, i.e., toward the "pith," we note large, circular, or angu- 

 lar cavities. The walls of these cells are quite thick and woody. They are 

 therefore called wood cells, and because they are continuous with cells above 

 and below them in the stem in such a way that long tubes are formed, they 

 are called woody vessels. Mixed in with these are smaller cells, some of 

 which also have thick walls and are wood cells. Some of these cells may 

 have thin walls. This is the case with all when they are young, and they 

 are then classed with the fundamental tissue or soft tissue (parenchyma). 

 This part of the bundle, since it contains woody vessels and fibres, is the 

 wood portion of the bundle, or technically the xylem. 



* This ring and the bundles separate the stem into two regions, an outer 

 one composed of large cells with thin walls, known as the cortical cells, or 

 collectively the cortex. The inner portion, corresponding to what is called 

 the pith, is made up of the same kind of cells and is called the medulla, or 

 Pith. When the cells of the cortex, as well as of the pith, remain thin-walled 

 the tissue is called parenchyma. Parenchyma belongs to the group of tis- 

 sues called fundamental. 



