XV.] THE BRACKEN FERN. 447 



of the bundle. Among the vessels a few parenchymatous 

 cells containing starch are scattered. The phloem is at 

 once distinguished from the xylem by the smaller average 

 size of its cells, and their thinner walls. The most con- 

 spicuous 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. Between the phloem and the 

 endodermis is a layer of parenchymatous 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 invested 

 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 continuous. 



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 adjacent 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 



