SPOROPHYTE. 23 



also straighten up their originally oblique wall, until it stands perpendic- 

 ular to the surface of the stem. Thus there results a basal cell of a hair, 

 with a hairless sister-cell beside it. A similar development of protective 

 ramentum was described by the writer on the stem-tip of Nymphaea. 



Returning to the plerome rudiment, it develops much more slowly than 

 the cortex. It divides periclinally into two equal parts (fig. 106) and each 

 of these again by similar walls, giving four layers of cells in the plerome. 

 Apparently the outermost and innermost of these give rise to pericycle and 

 endodermis, while the two median probably produce xylem and phloem. 

 Certain it is that all the tissues just named come from the four cells in 

 question. It is also certain that the endodermis is formed at the last peri- 

 clinical division in the outermost layer of plerome, and each endodermal 

 element is a sister cell to the pericyclic cell radially next to it. This has 

 been suspected by several writers for several ferns, by reason of the con- 

 tinuity of the radial walls. I was able to prove it for Dennst&dtia by find- 

 ing a number of mitoses (</. fig. 70, <5). 



The stem very quickly attains its final diameter; hence its broadly 

 rounded end. At a distance of 0.24 mm. from the initial cell, in an aver- 

 age instance, all the cells have reached their ultimate width. Already at 

 0.18 mm. in this case the epidermal cell- walls are thickening and becoming 

 dark brown. At 0.345 mm. the protophloem is mature and at 0.5 mm. 

 lignification is beginning in the xylem. In another case, however, the 

 first thickenings of xylem occurred at 6 mm. from the apex, and in another 

 at 0.18 mm. (cf. fig. 70). 



Excepting the epidermis, the first tissue to mature is the protophloem 

 (fig. 107). For some time it forms a prominent ring of small, angular, 

 thick- walled fibers near each boundary of the vascular bundle. The walls 

 stain deeply with haematoxylin. Seen on the side, the cells are very long 

 and slender, and the walls are peculiarly and irregularly pitted. The xylem 

 matures very irregularly. A cell here and another there become lignified, 

 apparently without any order (fig. 110). The larger tracheids precede 

 the smaller, the first ones being often near the exit of a root or leaf. No 

 spiral cells are formed, and there is nothing that could be called protoxylem. 



THE LEAF. 



In the taxonomic literature of ferns the chief attention is directed to the 

 leaf and its important appendages. Thus several references have already 

 been made to the leaf of Dennst&dtia (figs. 1,2). In the following descrip- 

 tion we shall designate as petiole, the leaf -stalk, from the rhizome to the 

 lowest pinna. All above this is blade. The continuation of the petiole in 

 the blade we shall call rachis, and the thin expansion of the leaf lamina. 



All of the leaves are alike in general appearance and in structure. There 

 is no morphological distinction of sporophyll and trophophyll, although for 

 various undetermined reasons some leaves are fertile and others sterile. 

 Both kinds are produced throughout the season, though the greatest devel- 



