in the * Seedlings' of Certain Leptosporangiate Ferns . 383 
are quite normal. The position of the latter, exactly opposite the radial 
walls of the pericycle cells, demonstrates the origin of the two layers from 
common mother-cells. The pericycle is two cell-rows deep, except at the 
protoxylem groups, where it is only one layer thick. Its cells have crowded 
contents and large nuclei. 
In the lower portion of the petiole there are two (or more, v. si) vascular 
strands, each possessing a curved xylem plate with generally a single 
protoxylem group. The large scalariform tracheides are surrounded by 
a layer of parenchyma cells with deeply-staining contents. The parenchyma 
is succeeded by the phloem, which is arranged in two chief masses, one on 
either side of the xylem plate ; the sieve-tubes are of the normal type and 
very prominent, staining intensely with haematoxylene, especially if subse- 
quently treated with ammonia. Round the ends of the xylem plate, the 
phloem is reduced to a single row of sieve-tubes intermixed with a few 
parenchyma cells (Fig. 204). The two bundles gradually approach one 
another, the first stage of the final fusion being the inclusion of the two 
strands within a common endodermis. The curved xylem bands are 
situated back to back, separated by phloem, and with the protoxylems 
widely diverging. A junction is gradually effected, resulting in a Y-shaped 
strand of xylem with a phloem patch in each of the three bays ; the 
protoxylems occupy the apices of the arms of the Y (Figs. 205-20 7). 
The actual junction of the bundles takes place at successively higher levels 
in the later leaves, and in large petioles there are from three to six bundles 
which are separate throughout. After the junction of the bundles in the 
earlier leaves, the endodermis cells become even more conspicuous than 
before, owing to the further thickening of their outer tangential walls. 
The thickening and lignification frequently extends also along the radial 
walls of the next outer layer of cells. 
Periderm . 
In one or two of the plants examined, frequent patches of periderm 
were noticed. As the cells outside the periderm were broken and dis- 
organised, there seems to be little doubt that the rhizome had been 
accidentally injured at these places, and that the injury had been followed 
by the formation of wound-periderm. As noted by De Bary 1 , periderm is 
of extremely rare occurrence in ferns, and, when present, generally seems to 
be connected with the presence of wounds. Periderm has been described 
by Holle 2 as occurring in the root and rhizome of Botrychium , and more 
recently, Brebner 3 finds wound-periderm of somewhat frequent occurrence 
in Marattiaceae. 
1 Comparative Anatomy, p. 108. 
2 Ueber Bau und Entwicklung der Vegetationsorgane der Ophioglosseen. Bot. Zeit., 1875. 
3 On the anatomy of Danaea and other Marattiaceae. Annals of Botany, xvi. 
