282 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
The trichoblast, even when formed, does not always become a 
trichome. It may remain short without putting out a hair, or it may 
become somewhat elongated and resemble an atrichomic cell. A 
singular outcome is to be seen in Nymphaea, in which trichoblasts 
which are not to be piliferous become suppressed and are overlaid 
by the conjoined ends of the elongated cells standing next above 
and below in the row. 
The base of the mature trichome is characteristically short when 
compared with the adjacent non-piliferous cells. It is often strongly 
wedge-shaped (Aneimia, Equisetum, tielaginella rupestris, Lycopo¬ 
dium, Sagittaria, Naias). Its inner wall is often pushed out under 
the extremities of the contiguous cells of the same row, as in many 
of the Helobieae, notably in Naias (pi. 17, fig. 42). In Naias, Po- 
tamogeton, and Cymodocea, Sauvageau has observed that the 
piliferous cells persist after the other external cells have fallen 
away. The base of the trichome sometimes presses in upon and 
displaces the underlying cortical cells, as in Limnobium iSponqia 
(pi. 17, figs. 49, 51). 
The nucleus, along with much protoplasmic matter, often lies in 
the basal part of the cell (pi. 17, figs. 48, 51). This is in contrast to 
the condition prevailing in trichomes of the first type, where, after 
the growth of the hair is completed, the nucleus takes up a position 
about midway of the tube, as a lenticular body adherent to the wall. 
The nucleus of the trichoblast may be very much larger than the 
nucleus of the surrounding cells, as in Limnobiiwn Spongia (pi. 17, 
fig. 49). The walls of hair-cells of this type are often darker 
colored than the walls of the non-piliferous cells. 
In those species in which the trichomes are of the specialized 
character described above, the epidermis of the root is thus composed 
of two very different elements. The development of this complex 
integument may be described as follows. Beginning with the parts 
nearest the apex, we have first , a short zone of cells undergoing 
equating divisions; second , a region in which by differentiating 
divisions the trichoblasts are cut off in a generally acropetal suc¬ 
cession ; third , a zone in which the atrichomic cells elongate and 
divide transversely (though division may be characteristically absent); 
in this zone the trichoblasts may become papillate; finally a fourth 
region, where, with the completion of division, the tissues of the root 
rapidly elongate, the root-hairs grow to their full size, and the layer 
becomes fixed. 
