262 



BOTANY 



The innermost layer of the cortex forms the endodermis, or the 

 bundle-sheath, whose radial walls are usually folded, giving the ap- 

 pearance, in transverse section, of dark spots. From special cells of 

 this layer, as already stated, the secondary roots arise. 



The root-bundle, as in other vascular plants, is of the radial type. 

 In the greater number of Ferns the bundle is " diarch " ; i.e. there 

 are two groups of xylem alternating with as many phloem masses. 

 Monarch roots occur in Ophioglossum vulgatum. Botrychium Virgini- 

 anum has usually tetrarch root-bundles, and in the larger roots of 



F. 



FIG. 227. Polypodium falcatum. Development of sporangium. A-E, from fresh 

 specimens (X 400). F, G, microtome sections (X 200). B, C, E, optical sections; 

 t, tapetal cells. 



Marattia and Angiopteris, the number of xylem and phloem masses 

 is much greater. 



Between the endodermis and the outer xylem and phloem elements 

 which it encloses, there is a layer of tissue, usually a single layer of 

 cells, the pericycle. As in the bundles of the stem, the primary 

 xylem-elements are small spiral or reticulate tracheids, and the 

 secondary ones larger scalariform elements developed toward the 

 centre of the bundle. The structure of the phloem is much like 

 that in the stem-bundles. 



The cortical part of the root is composed in part of parenchyma, 

 but the inner portion usually shows a greater or less development of 

 sclerenchyma. 



