346 



PART III. THE CLASSIFICATION OF PLANTS. 



spherical bodies, are developed in special receptacles (cupules) situated on the 

 dorsal surface of the apex of the shoots ; their mode of origin resembles that of 

 the gemmas of Marchantia. In most foliose forms the gemmae are developed 

 from marginal cells of the leaves (e.g. Jungermannia ventricosa, Scapania nemo- 

 rosa), or from cells near the growing- point of the stem (e.g. Jungermannia 

 bicuspidata). In these forms the gemmaa are usually uni- or bi-cellular, but in 

 Eadula complanata (where they are formed on the leaf-margin) they are flat 

 multicellular plates of tissue. 



The leaves are developed, generally speaking, one from each segment formed 

 at the growing-point. In tbe typical Acrogynw each dorso-lateral segment gives 

 rise to a lateral leaf, and each ventral segment to a ventral leaf (amphigas- 

 trium) ; though, as already mentioned, the amphigastria are wanting ia many 

 species. A characteristic feature of the leaves of this group is that they are 



FIG. 244. Branches of one of the acro- 

 gynous Jungermannieae, Plagiochila as- 

 plenioides, seen from above : the leaves are 

 succubous ; at the apex, two of the shoots 

 bear sporogonia, the one (b) having de- 

 hisced, the other (o) being still closed; p 

 the involucre. 



FIG. 245. Part of a shoot of Frul- 

 lania dilatata seen from below ( x 20) : 

 ul auriculate lower leaf-lobes ; ol upper 

 leaf -lobe; the leaves are incubous; u 

 amphigastrium. 



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 pre- 

 sents two forms : either the posterior edges of the leaves overlap the anterior 

 edges of those next bebind them (Fig. 244), 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. 245), when the leaves are said to be incubous. 

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



