14 STRUCTURE AND LIFE-HISTORY OF HAY-SCENTED FERN. 
so that the cell-walls are about one-fourth the width of the lumens, there 
are (fig. 44) two large tracheids slightly thickened, and two in the center 
wholly unthickened and containing protoplasm. Only with the decay and 
disruption of the outer layers of the root do the inner cortex and xylem 
become fully mature (fig. 45). This occurs from 3.5 cm. to 10 cm. from 
the root tip. One can not as a rule, obtain a transverse section showing 
all of the tissues mature and intact. 
These facts have an important physiological significance. We suppose 
the water-supply of the plant to come in through the root-hairs. But 
where these are functional the xylem, always considered the water-con- 
ducting tissue, is decidedly immature. It is evident that lignified walls are 
not necessary for the conduction of water in the cells. It may be, however, 
that the inner cortex, whose walls thicken at such an early period, is fora 
time active in water conduction. 
In 25 out of 95 roots examined (7. ¢., 26.3 per cent) from localities in 
Long Island, Pennsylvania, and Maryland, a more or less copious growth 
of non-septate fungus hyphe was found in the middle cortex (figs. 52, 
53). One of the most pronounced cases was attached to a rhizome of 
unusual thickness and width and with an unusually long and rapidly grow- 
ing meristematic apex. In a root-tip from this plant, hyphe were seen 
3.07 mm. from the initial cell. They were located in the epidermis and 
hypodermis, with branches running inward. In an older root, a hypha 
was seen extending into the root through a root-hair (fig. 53). Once in 
the middle cortex, strong hyphe run from cell to cell, ramifying in each 
cell to form a dense granular mass (fig. 53). Sometimes the fungus is 
found only on one side of the root, or in only a few cells. Occasionally 
it spreads all the way round. From the undoubted vigor of the host where 
the fungus occurs, the carly stage of the root at which it appears, and the 
mode of copious branching of the hyphe in the medio-cortex, we feel 
justified in considering that we have to do with a true mycorhiza. On 
account of its inconstancy, it may be called facultative. No other poly- 
podiaceous fern is known to possess such a commensalism, though Janse 
(1895) has recorded a similar condition in the aerial roots of Cyathea 
(species not given). 
Lateral rootlets arise from rhizogenous cells of the endodermis, opposite 
the xylem rays, as is universal in ferns. The endodermis has been shown 
(p. 10) to originate in the root apex with two cells in each segment, 7. e., 
one in each sextant. After the segment is divided transversely into two 
layers, there are two endodermal cells to a sextant, one lying nearer the 
apex than the other. In the two opposite sextants in which the protoxy- 
lems are later to form, the endodermal cell lying nearer the root apex 
remains undivided by any radial walls, while its posterior and sister cell, 
like all the other endodermal cells of the root, is halved radially. This 
large cell, extending clear across the sextant, is the primitive rhizogenous 
