41 6 PRTi^TAR}' ARRANGEMENT OF TISSUES. 



e.g. Fig. 154, or it only contains starch opposite the vascular bundles; e.g. in the 

 stem of Brassica oleracea ; or only opposite the medullary rays of the ring (shoots of 

 Atragene alpina). In the latter cases, the layer, in so far as it contains no starch 

 grains, is difficult to distinguish. 



A sheath developed as a starch-layer is present around the single bundle-trunks, 

 and then also embraces the accompanying fibrous strand, in the leaves of Dicoty- 

 ledons, and in the stem and sheath of the leaf of Monocotyledons (e. g. Grasses, as 

 Triticum, Zea). In the former cases it embraces the outer edge of the bundle, in 

 the Monocotyledons the inner (comp. Fig. 151, p. 331). 



As regards the sheaths of the ultimate ramifications and ends of the vascular 

 bundles, the latter, as already stated in Sect, iii, are surrounded in the foliar 

 expansions of Angiospermous plants by a layer of parenchymatous cells elongated 

 in the direction of the course of the bundle ; these cells are closely attached to the 

 bundle, and are laterally connected one with another without interruption. Their 

 surfaces remote from the bundle often border on the air-containing interstices of 

 the surrounding parenchyma, and where they abut on maiiy-armed lacunar tissue 

 their walls are often folded like those of the latter. In many leaves they resemble 

 the surrounding cells in containing abundant chlorophyll, e. g. Fuchsia, Papaver, 

 Primula sinensis, Zea, and Triticum (comp. Figs. 173, 175, 178, pp. 372-375); in 

 thick leaves they often contain little or no chlorophyll. 



