lO 



INTRODUCTION. 



investigated; e.g. species of Allium, and Canna, Hordcum vulgare, Triticum vulgare, 

 Zea INIais (Fig. 3), Stratiotes aloides, Alisma Plantago, Acorus Calamus, (Janczewski). 

 Treub ^, as the result of his extended researches, ascribes this differentiation to Jun- 

 caceae, Hacmodoracex, Cannaceae, Zingiberaceae, Typha, Cyperaceae, Gramineae, Com- 

 melynex, Potameae, Juncagineae, Sagittaria, Limnocharis, Stratiotes. But he differs 

 from Janczewski with regard to Allium, Acorus, and Alisma, since he does not allow the 

 presence of a calyptrogen-layer in the families Liliacea^, Astelieae, Xerotideae, Aspidis- 

 trcae, Ophiopogoneae, Amaryllidaceae, Hypoxideas, Dioscoreae, Taccaceae, Bromeliaceae, 

 IMusaceae, Orchidcae, Palmae, Pandaneae, Cyclantheae, Aroideae except Pistia, further in 



Fig. 3. — Median longitudin.il section through tlie apex of the root o( Zea Mats, from Sachs' Textbook, a— a outer, 

 i' inner layers of the root-cap; j calj'ptrog'en layer; ?«.^yyplerome ; .^rudiment of a vessel ; x, r — ?- periblem, or the 

 cortex which has developed from it ; e Epidermis, or dermatogen layer (w=:the thickened outer wall of its cells). Above 

 the apex of the plerome cylinder, easily seen between ?« and s, the dermatogen and periblem layers are reduced to two 

 initial layers, which occupy the depressed centre. According to Janczewski the initial group should consist of a single layer. 



the Iridaceae, Pontederiaceae, Sparganium, Butomus, and doubtfully in Alisma. He finds 

 rather, covering the sharply defined apex of the plerome, a group of common initial cells 

 two layers thick, from which originate root-cap, dermatogen, and periblem. Hence, the 

 last named families should represent a special type, differing to a certain extent from 

 those first named. 



^ M. Trcub, Le meristeme primitif de la racine dans les monocotyledones, Leiden, 1S76. 



