The stem of _Panicum fluitans_ is round in outline in the transverse section and has a large cavity. Just close to the cavity and separated from it by only one or two parenchymatous cells are found vascular bundles forming a series. Outside this series of bundles lies a sclerenchymatous band which is wavy, following the lower edges of the large air-cavities. One series of bundles is connected with this sclerenchymatous ring. The air-cavities are large and uniform and are separated by bands of parenchymatous cells. In each of these bands lies a vascular bundle on the upper side near the periphery. Sometimes we find, especially in young stages, diaphragms of stellulate cells stretched across the air-cavities. Later as the stem matures these disappear and the cavities become conspicuous. (See figs. 42 and 43.) =Structure of the root.=--As already stated, the roots of grasses conform to the monocotyledonous type, but the variations met with in their structure are not so great as in the case of the stem. The root-tips are protected by root-caps, and the actual tip of the root is very distinct in the roots of all grasses and it can be seen very clearly in a longitudinal section of the root. The actual tip of the root is sharply distinct from the root-cap as there are two distinct sets of cells, one giving rise to the root-tip and the other to the root-cap. The young root-tips are always free from root-hairs, and they are confined to the portions behind the root-tips. The extent of the root-hair region will vary according to the vigour and development of the roots and the nature of the soil. The root-hairs are mere protrusions of the cells of the outermost layer of the cortex of the root and this layer is called the =piliferous layer=. [Illustration: Fig. 44.--Transverse section of a part of the root of Pennisetum cenchroides. x 100 1. Root-hair; 2. piliferous layer; 3. cortex; 4. endodermis; 5. pericycle; 6. xylem; 7. phloem.] To learn the structure of the roots of grasses we may select as types the roots of _Pennisetum cenchroides_ and _Andropogon Sorghum_ and consider their structural details. In the transverse sections of these roots we find a fairly broad cortex consisting of thin-walled parenchymatous cells more or less regularly arranged. (See figs. 44 and 45.) Just below the piliferous layer two or three layers of thick-walled cells are seen. In the roots of _Andropogon Sorghum_ these thick-walled cells are very conspicuous as they consist of several layers. These layers of thick-walled cells constitute the =exodermis=. (See fig. 46.) The innermost layer of cells of the cortex is called the =endodermis= and it becomes conspicuous on account of the thickening in the lateral and inner walls of the cells of this layer. (See figs. 44 and 47.) [Illustration: Fig. 45.--Transverse section of the entire root of Andropogon Sorghum. x 25]