290 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
cells are considerably shorter than their atrichomic neighbors (pi. 17, 
fig. 27). The fully grown tube is long and slender. 
Under certain conditions of growth or environment of the root, 
no trichoblasts are produced. In such cases the roots remain without 
root-hairs. 
The following species were examined: Isoetes azorica , 1 ’ 
Bolanderi , I. Butler i, I. canadensis , I. Eatoni , I. echhiospora 
Braunii , 1. Echinospora rnuricata , I. Engelmanni , I. foveolata , I. 
Howellii , 1. hystrix , Z Kirkii , Z macrospora , Z melanopoda , Z 
melanospora , Z. Nuttallii , Z. Piperi , Z. riparia, Z. saccharata , Z 
Tuckermanni. 
Selaginella (pi. 17, fig. 28-30). — The differentiation of the 
trichomes, in their earlier as well as their later stages, is very marked 
in $. rupestris. In the fixed tissue of the epidermis the short hair- 
cells often alternate regularly with elongated hairless cells. The 
bases are a little wider than other cells of the row, and the hair is 
often borne somewhat laterally. In longisection the bases are 
characteristically more or less wedge-shaped (pi. 17, fig. 29). The 
hair is sometimes branched. 
The hair initials are determined in the meristematic tissue. In 
figure 28 (plate 17) the origin of the trichoblast is shown at T. An 
epidermal cell at the edge of the root cap has been divided near its 
proximal end. The narrower tabular segment, comprising about 
one third of the original cell, is the trichoblast. Both cells 
immediately elongate, decreasing meantime in radial diameter. In 
the tissue immediately succeeding them the short, dense trichoblasts 
stand out prominently amongst the more transparent elements. 
In most species of Selaginella examined the roots were poorly 
supplied with root-hairs. Neither a regular alternation of trichomic 
and atrichomic cells, nor the existence of cells in the young epider¬ 
mis destined to produce hairs could be demonstrated. The hairs 
stood singly. All the cells were relatively short, but nothing was 
seen to indicate that the origin of the trichomes is different from 
that in other species where they are plainly of the specialized type, 
e. g ., S. denticidata (pi. 17, fig. 30), S. Martensii , and S. viticulosa . 
Helobieae. 
Throughout this group trichoblastic hair-cells are characteristic. 
They are conspicuously different from the generally much elongated 
