LEAVITT: TRIG HOMES OF ROOT. 
289 
The fully formed trichome consists of a long and slender hair, and 
a sharply wedge-shaped base, intruded between two elongated 
epidermal cells and reaching usually somewhat more than half way 
to the inner plane of these cells. 
Phylloglossum. — Roots of Phylloglossum Drummondii from 
dried specimens treated with caustic potash show r ed the structure of 
the older epidermis well. Long hairless cells and short piliferous 
cells alternate in the longitudinal rows, as in Lycopodium. The root 
tips were not seen. 
Isoetes (pi. 17, fig. 23-27). — In Isoetes the production of 
trichomes is initiated by the formation of trichoblasts in the embry¬ 
onic region, near the apex of the root. The fact that the trichomes 
of the root of Isoetes are predetermined was stated by H. Bruchmann, 
in his paper, Ueber anlage und wachsthum der wurzeln von Lyco¬ 
podium und Isoetes (Jenaische zeitsch., 8: 522, 1874). 
The trichoblast may arise from the simple bipartition of a cell. In 
this case the dividing cell is cut, in general unequally, by a transverse 
wall perpendicular to the outer and inner faces. The upper or 
proximal segment becomes the trichoblast. It is readily distinguished 
from the surrounding cells in the living root by the somewhat 
greater refractive power of its contents. 
Generally, however, the process is somewhat more complicated. 
As a rule in the species examined, especially in freshly growing 
roots, the trichoblast is the proximal member of a group of four cells 
produced from an original unit by two successive divisions (pi. 17, 
fig. 23). That the trichoblast is from the moment of its inception a 
cell of specialized character is manifested by the fact that, while the 
second division gives in the lower half of the original cell two 
elements of the same size and appearance, in the upper half it results 
in unequal and dissimilar cells, of which the smaller and denser is 
the trichoblast (pi. 17, fig. 23, T ; fig. 24). 
The atrichomic cells of the group may, and generally do, remain 
undivided. Then the trichomes ultimately stand in the mature 
epidermis at set intervals, being separated in the rows by 3 non- 
piliferous cells (pi. 17, figs. 25, 26). But sometimes the intervening 
cells are increased bv further division so that the trichomes are 
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separated by 4 or more cells in the row. 
In rapidly growing roots the trichoblasts become papillate 
immediately back of the embryonic area. At maturity the trichomic 
