326 



PART IV. CLASSIFICATION". 



are wanting in many species. A characteristic feature of the leaves of 

 this group is that they are distinctly bilobed, at least when young ; this 

 is due to the fact that the mother-cell of the leaf is divided into two which 

 give rise to the two lobes. The leaves are sessile, and their insertion is at 

 first transverse to the long axis of the stem, so that one lobe is superior 

 or dorsal, the other inferior or ventral ; but by subsequent displacement 

 it becomes oblique. Since the leaves are situated close together, they thus 

 come to overlap each other, and this overlapping presents two forms: 

 either the posterior edges of the leaves overlap the anterior edges of those 

 next behind them (Fig. 201), when the leaves are said to be succubous ; or 

 the anterior edges of the leaves overlap the posterior edges of those next 

 in front of them (Fig. 202), when the leaves are said to be incubous. The 

 growth of the leaf is generally apical at first, and subsequently basal. 



Fie. 201. Brooches of one of the aero- 

 rrrnous Junxermanniacese, P'a/iiochila as- 

 flenioides, seen from above: the leaves are 

 succubous ; at the apex, two of the shoots 

 benr sporopon'a, the one (b) having de- 

 l.isced, the oiher(n) being still closed; p 

 the involucre. 



Fm. 202 Part of a shoot of Frul- 

 lania dilatata seen from below (x 20): 

 ul auricnlate lower leaf-lobes ; ol upper 

 leaf-lobe; the leaves are incubous; u 

 amphigastrium. 



The sexual organs are generally borne on the main axis and its normal 

 branches, but in many cases they are confined to more or less specialised 

 ventral branches (gametophores). The place of development of the arche- 

 gonia affords the basis for the classification of the Jungermanniacese into 

 the two main groups, Acrogynee and Anacrogynse. In the former, which 

 includes nearly all the foliose forms, the archegonia are produced 

 from the apical cell and its youngest segments at the growing-point ; 

 hence when the formation of the archegonia takes place on a shoot its 

 further elongation is arrested. In the latter group, which includes all 

 the thalloid forms and some exceptional foliose forms, the archegonia are 



