HISTOLOGY 



33 



en 



vascular bundles, some small and others large, pass from the 

 base to the apex. Externally the leaf is covered on both the 

 sides by the epidermis. The spaces existing between the vascular 



bundles and the epi- 

 dermis are filled with 

 parenchymatous cells. 

 The larger vascular 

 bundles consist of 

 xylem and phloem sur- 

 rounded by a bundle 

 sheath of a single layer 

 of cells. In the smaller 

 bundles the xylem is 

 very much reduced. 

 Around every vascular 

 bundle there is a 

 single row of somewhat 

 large cells densely 

 packed with large chlo- 

 roplasts, the chloro- 

 phyllous layer. The 

 vascular bundles are 

 strengthened by fibres, 

 on both the sides in the 

 case of larger bundles 

 and on only one side in 

 small bundles. 



Fig. 47. — Transverse section of the stele portion of 



the root of Andropogon Sorghum, x 150 



Co. Cortex ; en. endodermis ; pe. pericycle ; ph. 



phloem ; pr. protoxylem ; xy. xylem vessel. 



For a detailed study of the structure of the leaves of grasses 

 the leaf of the grass Panicum javanicum may be chosen. In a 

 transverse section of this leaf, the vascular bundles are very 

 conspicuous. The larger bundles are normal in every way, while in 

 the smaller ones the xylem elements are considerably reduced. 



FlG. 48. — A portion of the transverse section of the leaf of Panicum javanicum. x 100 



Ep. c. An ordinary epidermal cell ; st. stomata ; sc. sclerenchyma ; ph. phloen ; 

 chl c. chlorenchyma ; m. motor cells ; xy. xylem. 



Around every one of the vascular bundles there is a single row of 

 large cells containing large chlorophyll grains (the chlorophyllous 

 layer). In a well developed large vascular bundle the chlorophyl- 

 lous layer is open below just close to the sclerenchymatous band. 

 On both sides of the larger vascular bundle there are bands of 

 sclerenchyma. In the case of smaller bundles some are streng- 

 thened by sclerenchyma on the lower side and others have none. 

 The spaces between the bundles are occupied by thin-walled 

 parenchymatous cells containing small chlorophyll grains. 



