328 PART IV. CLASSIFICATION. 



layers (e.g. Pellia, Aneura) in the middle line of the shoot ; in those which 

 have a well-defined midrib (e.g. Metzgeria), the midrib consists of several 

 layers of cells, whereas the lamina consists of only a* single layer. In 

 Symphyogyna and Blyttia the midrib is traversed by a strand of elon- 

 gated prosenchymatous cells having thickened and more or less pitted 

 walls : a similar tissue occurs in the thick central portion of the shoot of 

 Pellia. 



In the Acrogynse, the stem generally consists of an axial strand of 

 relatively thin-walled cells, surrounded by a cortex of narrow thick-walled 

 cells : the leaves are simply single layers of similar cells, and have no 

 midrib. 



The root-hairs are, in all cases, destitute of the peculiar thickenings so 

 characteristic of the Marchantiacese. 



B. The SPOKOPHYTE. The course of the development of the sporophyte 

 is, in its main features, essentially the same throughout the Jungerman- 

 niacese. 



The oospore is divided by a transverse (basal) wall into two halves, 

 epibasal and hypobasal. The epibasal cell gives rise to the capsule and 

 its stalk (seta). It divides transversely, and the longitudinal divisions 

 follow in both cells so that the epibasal half of the embryo consists of two 

 tiers of each consisting of four cells. Further growth in length is effected 

 by the cutting off, by transverse walls, of segments from the cells forming 

 the apical tier ; but this apical growth is arrested, sooner or later, by the 

 formation of walls parallel to the free surface (periclinal) in the apical 

 cells, and also frequently in some of those below them, which indicate the 

 differentiation of the capsule-wall (amphithecium) from the internal mass 

 of cells (endothecium) which give rise to the spores and elaters. The 

 cells balow the capsule may, however, continue to grow and divide 

 transversely, and by means of this intercalary growth the full length of 

 the seta is attained; 



In many of the Jungermanniacese (e.g. Pellia, Jungermannia, Frullania) 

 the lower end of the seta developes into a bulbous mass of cells forming a 

 false foot, the upper margin of which grows up so as to form a sheath round 

 the lower part of the seta in some cases. 



The development of the hypobasal portion of the embryo is compara- 

 tively insignificant , in most cases it is merely a small appendage to the 

 lower end of the seta. The hypobasal cell enlarges somewhat, without 

 undergoing any division (e.g. Radula, often in Pellia); or it undergoes 

 transverse division to form a filament of two or three cells, the lowest of 

 which becomes elongated and grows down among the cells at the base of 

 the archegonium (e.g. Metzgeria, Aneura). In some forms, however (e.g. 

 Fossombronia), the hypobasal cell appears to give rise to a true foot, 

 bulbous in form, C9mparable to that of the Marchantieae (see p. 323). 



In the further differentiation of the capsule, the cells of the amphithe- 

 cium undergo periclinal division so that the wall eventually consists of 

 two or more (up to six) layers of cells. In the wall-cells transverse 

 annular thickenings are usually developed. The planes of dehiscence of 

 the capsule, except in those forms which dehisce irregularly (Riella, 



