41 8 , BOTANICAL GAZETTE [December 



The full grown hair may attain a length of about 2'"'". Its 

 short base and the position with respect to the other elements 

 of the epiblema, which it occupies in consequence of its peculiar 

 origin, give this sort of trichome a distinctive character {Jig, id). 

 The fact will be appreciated upon comparison of the figures 

 accompanying these notes with the root-hairs of ordinary 



dicotyledonous plants. 



The superficial layer of the root trunk in A. filicidoides and A. 

 caroli?iia?ia mdi,y h^ described as usually comprising, apart from the 

 apical cell, four regions. Beginning with the youngest, these 

 are: (l) a region of embryonic tissue in which the divisions 

 are equating divisions; (2) a short zone where the divisions are 

 differentiating divisions, giving rise ultimately to two sorts of 

 members, trichomes and flat or prismatic cells; (3) a more 

 extended belt, in which the cells of the second class again 

 undergo equating divisions, and elongate ; (4) a region of 

 matured and fixed tissue, covering the greater part of the root. 

 These regions represent successive stages in the genesis of 

 the layer under discussion. An epiblema with such a complex 

 history is characteristic not only of AzoUa — a highly special- 

 ized, terminal group — but of wide ranges of fern allies and 

 monocotyledons, and of an isolated group of dicotyledons. My 

 attention was first drawn to the matter by the similarity of 

 Sagittaria and Nymphaea in this respect. I find a similar type 

 of epiblema in many members of Juncaginaceae, Potamogetona- 

 ceae, Aponogetonaceae, Naiadaceae, Alismaceae, Hydrocharita- 



■ 



, Juncaceae, Cyperaceae, Gramineae, Commelinaceae, 

 Xyridaceae, Eriocaulaceae, Haemodoraceae, Zingiberaceae, 

 Marantaceae, Orchidaceae ; and in Nymphaeaceae (Nymphaea, 

 Nuphar, Brasenia, Cabomba) alone among the dicotyledons 

 examined. That the same trait is exhibited by such divergent 

 groups as Lycopodium Isoetes (already noted by Bruchmann), 

 Selaginella, and Equisetum — the last two instances seem to 

 have been overlooked — shows that we have to do with an old 

 element in vascular plants. 



The Ames Botanical Laboratory, 



North Easton, Mass. 



