PATH OF MOVEMENT. 45 



situated at the same distance from the center of the stem as the middle part 

 of the bundles, and that it divides the bundles into two groups of cells. 



99. 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. 



100. Bast portion of the bundle. If our section is through a part of the 

 stem which is not too young, the tissues of the outer part of the bundle will 

 show either one or several groups of cells which have white and shiny walls, 

 that are thickened as much or more than those of the wood vessels. These 

 cells are bast cells, and for this reason this part of the bundle is the bast por- 

 tion, or the phloem. Intermingled with these, cells may often be found which 

 have thin walls, unless the bundle is very old. Nearer the center of the 

 bundle and still within the bast portion are cells with thin walls, angular and 

 irregularly arranged. This is the softer portion of the bast, and some of 

 these cells are what are called sieve tubes, which can be better seen and 

 studied in a longitudinal section of the stem. 



101. Cambium region of the bundle. Extending across the center of the 

 bundle are several rows of small cells, the smallest of the bundle, and we can 

 see that they are more regularly arranged, usually in quite regular rows, 

 like bricks piled upon one another. These cells have thinner walls than any 

 others of the bundle, and they usually take a deeper stain when treated 

 with a solution of some of the dyes. This is because they are younger, and 

 are therefore richer in protoplasmic contents. This zone of young cells 

 across the bundle is the cambium. Its cells grow and divide, and thus increase 

 the size of the bundle. By this increase in the number of the cells of the 

 cambium layer, the outermost cells on either side are continually passing 

 over into the phloem, on the one hand, and into the wood portion of the 

 bundle, on the other hand. 



102. Longitudinal section of the bundle. If we make thin longisections of 

 the vascular bundle of the castor-oil seedling (or other dicotyledon) so that we 

 have thin ones running through a bundle radially, as shown in fig. 42, we 

 can see the structure of these parts of the bundle in side view. We see here 

 that the form of the cells is very different from what is presented in a cross 

 section of the same. The walls of the various ducts have peculiar markings 

 on them. These markings are caused by the walls being thicker in some 



