288 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
teristic of some other groups, for example, some Monocotyledons, 
where it occurs along with occasional longitudinal splitting of the 
trichoblasts (pi. 18, fig. 86). 
A single case of transverse division of a trichoblast was seen. 
The atrichomic cells seem rarely, if ever, to divide transversely after 
the trichome initials appear. The cutting off of trichoblasts may be 
intermitted. Generally, however, the trichomes are produced very 
abundantly. 
Lycopodium lucidulum (pi. 16, fig. 20-22). — The earliest rudi¬ 
ments of the trichomes lie close to the apex, under the root-cap. 
The transverse wall, separating the newly formed trichoblast from 
the atrichomic sister segment of the original tabular epidermal cell, 
is oblique and curved. Beginning near the inner edge of the prox¬ 
imal end wall of the mother cell, this membrane of division runs 
outward at an acute angle with that wall, toward the external face 
of the mother cell. At length it curves toward the root apex, and as 
seen in longisection meets the outer wall usually a little forward of 
the middle point (pi. 16, fig. 20, T). In tangential section, also, the 
new wall is curved, with the convexity toward the root apex (pi. 16, 
fig. 21). 
The lenticular, wedge-shaped, proximal segment thus cut off is 
from the first distinguished by relatively dense contents. This cell 
is the trichoblast. As the elements of the layer increase in radial 
and longitudinal dimensions, the atrichomic cells outstrip the 
trichoblasts. The latter soon reach full size, while the intervening 
cells continue to elongate. Immediately back of the root-cap the 
outer face of the trichoblast becomes protuberant. The further 
extension of this wall in tubular fashion, in adaptation to the absorp¬ 
tive function of the cell, gives rise to a hair which at first is inclined 
somewhat forward, toward the root apex (as in Lycopodium inun- 
datum , Equisetum, and Azolla). 
Sooner or later the trichoblast generally divides longitudinally 
once (pi. 16, fig. 22) ; or, less frequently in this species, twice 
successively. The trichomes are therefore to be found generally 
side by side in pairs, or more rarely in groups of four. If no longi¬ 
tudinal splitting occurs, the single hair arises aside from the median 
position, as in L. inundatum. Not infrequently in the material 
examined, cases were found where two trichoblasts had been formed 
successively in the row. 
