14 



near its base small prickle sliaped expansions and two-celled hairs 

 occur. Stomata are numerous over the chlorophyll tissue and are 

 unprotected. They are not all depressed below the surface of the leaf, 

 and the guard cells are covered with cuticle that is only half as thick 

 as that over the other epidermal cells. 



The arrangement of chlorophyll-bearing cells is concentric with 

 respect to each fibro-vascular bundle, and each bundle Avith its encir- 

 cling rows of chlorophyll-bearing cells is entirely separated from the 

 adjacent concentric systems by bands of colorless parenchyma that 

 extend from the bulliform cells above to the large epidermal cells 

 below. 



The fibro-vascular bundles of the primary order have an unbroken 

 sheath of from one to several rows of cells. The sheath cells below the 

 bundle are often in two or more rows and have a small excentric himen 

 and very thick walls. The upper cells sometimes have comparatively 

 large central cavities and thin walls, aud again are like the lower cells. 

 In the lower cells the lamination of the cell walls and the radial canals 

 tlirough them are conspicuous. Bundles of the secondary order (one 

 to three of which intervene between two primary ones) have no large 

 oval vessels and no definite line between xylem and phloem. Their 

 sheath is of a single row of thick-walled cells below the bundle and 

 sonu'tinies on the sides, but above it is replaced by a group of thin- 

 walled, large-lumened cells, which, together with the bundle, assumes a 

 sharply oval or pear-shaped form. In longitudinal section the length 

 of these cells is from 1^ to 3 times their diameter. The transverse 

 walls are oblique and the ends overlap. The walls are often deli- 

 cately pitted. Every bundle is encircled by a ring of chlorophyll- 

 bearing cells (fig. 1, ch, ri. VILl) that are flattened on the sides next 

 each other and toward the bundle, but are convex on the outer side of 

 the ring. Separation of these cells by maceration proves that they are 

 closely coherent in longitudinal rows and that the convex wall (fig. 10, 

 ch, PI. IX) is of such uneven thickness that it may be said to be either 

 deeply pitted or reticulately thickened, while the other walls show 

 neither pits nor reticulate markings, though they are very firm in con- 

 trast to the delicate cell walls of the rest of the chlorophyll-bearing 

 tissue. This last, like that of nearly all grasses whose tissues have a 

 similar concentric arrangement, is composed chietiy of cells whose long 

 axis is parallel with that (^f the leaf and at least two of whose sides are 

 marked with dee[) regular incisions that are opposite each other aud 

 divide the cell into regular lobes (fig. 10, / />, PI. IX). The greatest 

 width of the cell in a direction radial with respect to the bundle is 

 several times greater than the width in a tangential direction, so that 

 in cross section the cells seem to be radially arranged about the inner 

 ring of pitted chlorophyll bearing cells. This arrangement is very 

 clearly seen in bleached sections (fig. 9, PI. IX). The bands of colorless 

 parenchyma (fig. 1, c^;, PI. VIII) between the ridges consist of one or 



