XV.] THE BRACKEN FERN. 447 



cells, and their thinner walls. The most conspicuous elements 

 of the phloem are the sieve-tubes, which consist of long cells, 

 the lateral walls of which show groups of minute perforations. 

 The outer layer of the phloem is formed of long narrow cells, 

 with comparatively thick walls. These cells, are developed 

 before the rest, and are hence termed the protophloem. Be- 

 tween the phloem and the bundle-sheath is a layer of paren- 

 chymatous cells containing starch, termed the phloem- 

 sheath. 



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, 

 interrupted by irregular paler markings, which are the trans- 

 verse sections of longitudinal bands of a similar colour. 

 There are no brown spots or bands. Examined micro- 

 scopically, the ground-substance is found to be composed 

 of polygonal cells containing chlorophyll. These are in- 

 vested superficially by an epidermis, composed of elongated 

 cells. The pale bands are vascular bundles, of similar 

 structure to those in the stem, with which they are con- 

 tinuous. 



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 com- 

 ponent cells become somewhat more irregular. On the 

 under side, many hairs are developed from it, and the 

 cells become singularly modified in form, their walls being 

 thrown out into lobes, which interlock with those of adja- 

 cent cells. 



Between certain epidermal cells on the under surface an 

 oval space is left, forming a channel of communication 

 between the interior of the frond and the exterior. The 



