GROUP II. BHYOPHTTA : HEPATIC^E. 327 



produced laterally, on the dorsal surface in the dorsiventral forms, on all 

 sides in the radial forms (species of Biella, Haplomitrium) ; hence the 

 growth in length of the shoot is not necessarily arrested. 



The archegonia of the Acrogynae are borne eithe'r singly or in groups of 

 two or more. Each archegonium is developed from a single cell; when 

 the archegonium is single it is developed from the apical cell ; when there 

 are several archegonia, the development of them begins in the youngest 

 segment-cells of the growing-point. The archegonia are surrounded by 

 the leaves of the apex ; and in most cases the leaves of the last whorl are 

 coherent, forming an involucre, surrounding the single archegonium or 

 the group of archegonia. 



The archegonia of the thai lo id Anacrogynae are borne in median dorsal 

 groups : in the radial Anacrogynae (Riella helicophyUa, Haplomitrium) 

 they are borne singly, scattered over the whole length of the stem as in 

 the former, or confined to the apical region as in the latter. They are in 

 all cases provided with some sort of protective organ. Among the thalloid 

 Anacrogynae the group of archegonia is surrounded, in Metzgeria, Aneura, 

 and Pseudoneura, by an involucre consisting of the short modified game- 

 tophore (ventral in Metzgeria) ; in Pellia, Symphyogyna, and Sphaero- 

 carpus a group (or each archegonium as generally in Sphaerocarpus) is 

 surrounded by an involucre developed as an outgrowth of the tissue of 

 the fertile branch. 



The antheridia are borne, in all Jungermanniacese (except Haplomitrium) 

 on the dorsal surface of the shoot ; in Haplomitrium they are borne in 

 three rows on the sides of the apical region. 



The antheridia are shortly stalked and are in all cases provided with 

 a protection. In Metzgeria the group of antheridia is invested by an 

 involucre which consists of the short modified ventral gametophore : in 

 the other thalloid Anacrogynae (e.g. Pellia) each antheridium is invested 

 by an involucre which grows up around it, so that it appears to be sunk 

 in the tissue of the shoot. In the Acrogynae the antheridia are borne, 

 singly or several together, in the axils of leaves; and in some forms 

 (e.g. Scapania, Lejeunia, Frullania) the upper lobe of the protecting leaf 

 is modified in form. 



The distribution of the sexual organs varies even in the species of some 

 of the genera. Some of the Anacrogynse (e.g. Metzgeria, Pseudoneura, 

 Sphaerocarpus, Haplomitrium, etc.) are dioecious ; whereas others (e.g. 

 Pellia, some species of Aneura, Fossombronia, Symphyogyna) ar gener- 

 ally monoecious. In the monoecious forms the antheridia and archegonia 

 are generally borne on distinct branches (diclinous), but sometimes on the 

 same branch (monoclinous). When a dorsiventral shoot bears only 

 antheridia or archegonia, they are developed in the median line; but 

 when it bears both organs, the archegonia are median and the antheridia 

 lateral. 



The Acrogynae are generally monoecious and diclinous. 



The structure of the adult shoot of the Jungermanniaceae is very simple. 

 In thalloid forms which have no distinct midrib, the shoot consists of 

 parenchymatous cells forming a single layer at the margin and several 



