292 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
given for the hair-cell of Limnocharis emarginata. Trichomes and 
elongated cells often alternate with entire regularity. The prismatic 
non-piliferous cell may, however, divide transversely once or twice. 
The trichoblast may mature as a hairless short cell. Trichoblasts 
may be absent for a time under conditions, as we may suppose, not 
calling for absorptive activity on the part of the root. 
The following species belonging to the Helobieae have been 
examined. The hair-cells arise as trichoblasts. 
Juncaginaceae : Triglockin maritima (pi. 17, fig. 34). This example 
was also noted by Juel ( l . c.). 
Potamogetonaceae: several native species of Potamogeton were 
investigated. Trichomes are abundant, with bases only one 
fifth or one sixth the length of the non-piliferous cell. The 
walls are often dark colored. Sauvageau 1 notes the peculiari¬ 
ties of the trichomes in the European species seen by him and 
says that those superficial cells which bear the hairs persist 
longer than their congeners. Ruppia 'maritima (pi. 17, fig. 
35). Zannichellia palustris (pi. 17, fig. 36). Zoster a marina. 
Trichoblasts are formed. Sauvageau, in the paper above cited 
observes that the peculiar piliferous cells persist after the fall 
of the other epidermal cells. 
Cymodoceae: Sauvageau has called attention to a differentiation of 
piliferous and non-piliferous cells in the epidermis in this 
group. 2 The structure is very much like that observed by him 
in Naias. “A longisection of the tip of the root shows that at 
first the cells of the piliferous layer are of equal size, but they 
are soon differentiated, on the one hand into cells which remain 
narrow (short), become conical and develop into hairs, and on 
the other hand into cells which follow the elongation of the 
root and do not produce hairs.” The latter soon fall off, while 
the trichomes persist for a longer time. 
Aponogetonaceae : Aponogeton distachywn, Aponogeton ( Ouviran- 
dra ) fenestrate. 
Naiadaceae: Naias gracillima (pi. 17, fig. 37-40). The mature 
epidermis consists of exceedingly elongated cells — the ratio of 
1 Sauvageau: Journ. de bot., 3: 169 (1889). 
2 Sauvageau: Sur la racme des Cymodoceae. Association fran^aise pour 
ravancement des sciences, 20e session, p. 473 (1891). 
