12 



STRUCTURE AND LIFE-HISTORY OF HAY-SCENTED FERN. 



the second by Van Teighem and Dotiliot in 1888. The latter authorities 

 indicate that in some ferns {Ptcris, Adiantum, Aneimia, etc.) the outer 

 cell (figs. 14, 16, 17) may give rise to two or three layers of cortex in ad- 

 dition to the epidermis (see table of cell-lineage in fern roots, p. 46). Such 

 ferns are in the minority. The same writers state that in Rqnisetum, 

 Osmunda, and Todea the first periclinal wall is between the central cyl- 

 inder and cortex, but that this is not the case in any other Pteridophytes 

 which have a single initial in the root. 



I have found the account here given for Dennstcedtia as to the origin of 

 epidermis, hypodermis, cortex, and endodermis to apply equally to root-tips 

 of Cibotium regale, Aspidium molle (fig. 48), Lygodium japonicum, Onoclea 

 sensibilis (fig. 51), Ceratopteris thalictroides (fig. 50), and Aspidium viar- 

 ginale (fig. 47). In Pteridium aqnilinum and Didymockl&na lunulata (fig. 

 49) the epidermis and two layers of cortex are derived from the same part 

 of the segment. 



Above the region of cell division in the root-tip there intervenes a brief 

 region of elongation. Beyond this, viz, about 2.5 mm. from the apex, 



root-hairs appear. Each hair is 



... -, . ,1 /- ^1 TABLE 3. Root-hairs. 



a cylindrical outgrowth from the 



lower (distal) end of an epidermal 



cell. The cavities of cell and hair are 



continuous, and contain but one 



nucleus (fig. 252) lying variously 



in the wall-layer of protoplasm. 



In functional hairs the nucleus is 



seen near the swollen apex. The 



walls of the hairs are thin, of a clear, brownish-yellow color, and are often 



molded around irregular particles of earth. Table 3 gives the exact 



dimensions . 



A transverse section of the region of functional root-hairs (figs. 34, 35, 

 44) shows the epidermis, hypodermis, four or five (rarely three) layers of 

 cortex and a well-defined endodermis. A single layer (rarely doubled in 

 places) of pericycle surrounds the cylindrical, diarch bundle. Protoxylems 

 abut directly upon the pericycle at diametrically opposite points, and 

 between them lies a group of two to four (rarely five) large tracheids. 

 Extending around within the pericycle from each side of each protoxylem 

 is a row of three to seven sieve-tubes. Midway between the protoxylems 

 and lying against the pericycle is the small-celled, dense protophloem. 

 Between the phloem and xylem are cells of conjunctive parenchyma. 



The epidermis (piliferous layer of Van Tieghem, etc.) at the level we 

 are speaking of is fully mature, and consists of cells four to six times as 

 long as wide. In cross-section they are nearly isodiametric, of slightly 

 variable depth and width, and bulging out a little on the outer side. The 



