﻿of North America. 3 



dichotomously branched, with, or more frequently without, a well- 

 defined costa, usually of several layers of cells, often with interior 

 mucilage cavities, more rarely with intercellular air-spaces, without 

 proper epidermis, but provided on the ventral surface, sometimes 

 also on the dorsal, with inconspicuous clefts (" stomata," " muci- 

 lage-slits "), the thallus becoming through these infected with 

 Nostoc colonies; cells with a single large chloroplast commonly 

 enclosing the nucleus. 



Sexual organs embedded in the thallus; antheridia arising 

 endogenously, short-stalked, single or in groups of 2-4 (rarely 

 more), occupying cavities separated trom the dorsal surface by two 

 layers of cells, the covering ruptured at maturity; walls of arche- 

 gonium confluent with surrounding cells of the thallus, the neck- 

 canal communicating with dorsal surface at maturity of the egg- 

 cell. Special calyptra not differentiated. " Involucre " tubular, 

 of several layers of cells, formed from archegonium-wall and 

 adjacent cells of thallus, usually soon broken through at the apex 

 by the elongating capsule and remaining as a sheath about its 

 base, or (in Notothylas) irregularly torn by external agencies. 



Sporogonium consisting of a pod-like, usually erect and much 

 elongated capsule, a bulbous foot, and a short intervening zone 

 occupied by a long-active meristematic tissue ; capsule dehiscing, 

 with rare exceptions, from the apex downward by two valves, its 

 walls containing chlorophyl and in most species of Anthoceros 

 bearing stomata, each with two crescentic guard-cells ; a slender 

 thread-like columella, surrounded throughout and covered at the 

 apex by the spore-forming layer, probably always present, but some- 

 times early disintegrated and obscure. Spores more or less plainly 

 tetrahedral, long adhering in fours, smooth, verrucose, papillate, or 

 echinulate, ripening successively from apex of capsule downward ; 

 sterile cells from the archesporium single and subcubical at ma- 

 turity, or, more often, elongated and forming heteromorphic, vari- 

 ously contorted, occasionally branched filaments of 2-4 cells, 

 sometimes [Anthoceros sp. and Dendroceros — in tropics and southern 

 hemisphere) with one or more well developed spiral bands, or, 

 more commonly (pseudo-elaters), with spiral thickenings rudimen- 

 tary or wanting. 



Three genera, Anthoceros, Dendroceros, and Notothylas, are or- 



