VII.] THE BKACKEN FERN. 57 



walls simulate the rounds of a ladder, they have been termed 

 scalariform ducts or vessels. The cavities of these scalariform 

 ducts are divided at intervals, in correspondence with the 

 lengths of the cells of which they are made up, by oblique, 

 often perforated, partitions. Among the smaller vessels, a few 

 will be found, in which the thickening forms a closely wound 

 spiral. These are spiral vessels. 



The rachis of a frond, so far as it projects above the 

 surface of the ground, is of a bright green colour; and, in 

 transverse section, it presents a green ground-substance, inter- 

 rupted by irregular paler markings, which are the transverse 

 sections of longitudinal bands of a similar colour. There are 

 no brown spots or bands. Examined microscopically, the 

 ground-substance is found to be composed of polygonal cells 

 containing chlorophyll. These are invested superficially by 

 an epidermis, composed of elongated cells, with walls thick- 

 ened in such a manner as to leave thin circular spots here 

 and there. Hence, those walls of the cells, which are at 

 right angles to the axis of vision, appear dotted with clear 

 spots ; while, in those walls of which transverse sections are 

 visible, the dots are seen to be funnel-shaped depressions. 



The pale bands are vascular bundles containing scalari- 

 form and spiral vessels. The outer layer investing each 

 is chiefly formed of long hollow fibres with very thick 

 walls, and terminating in a point at each end. These scleren- 

 chymatous fibres have oblique cleft-like clear spaces, pro- 

 duced by interruptions of the process of thickening in their 

 walls. 



The vascular bundles, the green parenchyma, and the 

 epidermis are continued into each pinnule of the frond. 

 The epidermis retains its ordinary character on the upper 

 side of the pinnule, except that the contours of its component 

 cells become somewhat more irregular. On the under side, 

 many hairs are developed from it, and the cells become 



