=97.= We thus see that instead of the liquids passing through the entire stem they are confined to definite courses. Now that we have discovered the path of the upward movement of water in the stem, we are curious to see what the structure of these definite portions of the stem is. [Illustration: Fig. 58. Xylem portion of bundle. Cambium portion of bundle. Bast portion of bundle. Section of vascular bundle of sunflower stem.] =98. Structure of the fibrovascular bundles.=—We should now make quite thin cross-sections, either free hand and mount in water for microscopic examination, or they may be made with a microtome and mounted in Canada balsam, and in this condition will answer for future study. To illustrate the structure of the bundle in one type we may take the stem of the castor-oil bean. On examining these cross-sections we see that there are groups of cells which are denser than the ground tissue. These groups correspond to the colored areas in the former experiments, and are the vascular bundles cut across. These groups are somewhat oval in outline, with the pointed end directed toward the center of the stem. If we look at the section as a whole we see that there is a narrow continuous ring[7] of small cells 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 angular 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_.