EMBRYOGENESIS IN THE BRYOPHYTA 73 



transverse walls. Median longitudinal walls are then formed, especially 

 in the epibasal region, Fig. 15f. Thereafter, the divisions in the epibasal 

 region are periclinal, an amphithecium, or capsule wall, and an 

 archesporial endothecium being differentiated. As in the Marchantiales, 

 the epibasal segment of the first zygotic division gives rise to the capsule 

 and seta, while the hypobasal segment forms the foot. The genus 

 Porella, Fig. 1 5e, f, exemplifies the filamentous type of embryo in which 

 the subsequent divisions are of a somewhat irregular nature. 



POST-FERTILISATION DEVELOPMENTS IN THE GAMETOPHYTE 



In the bryophytes, the dependence of the sporophyte on the gameto- 

 phyte is usually, and rightly, stressed. The movement of metabolic 

 substances, however, is not all in the acropetal direction: the developing 

 embryo may have important effects on the adjacent gametophyte 

 tissue. Indeed, in some species, conspicuous and biologically important 

 structural changes are induced in the gametophyte. The renewed 

 growth of the ventral region of the archegonium after fertilisation is 

 general in bryophytes, a protective calyptra being thus formed. The 

 adjacent tissues of the thallus may also be stimulated to grow. In 

 Corsinia and Boschia, Fig. 16a, b, a protective shield-like investment 

 grows up from the thallus and covers the developing sporogonium. In 

 the Jungermanniales, e.g. Pallavicinia, the embryro is first of all 

 protected by a considerably enlarged and elongated perianth: this is a 

 gametophyte structure which has undergone further growth as a result 

 of the stimulus of the embryonic development. Examples of remarkable 

 post-fertilisation gametophytic developments are found in those 

 Jungermanniales in which a 'pouch' or marsupium is present, Figs. 

 16c-G. As the illustrations show, the ensuing morphological changes 

 are of a far-reaching kind, in which the embryo becomes protected in a 

 characteristic pouch more or less deeply embedded in gametophyte 

 tissue. Among the mosses. Sphagnum and Andreaea show an interesting 

 post-fertiUsation gametophyte reaction in the development of a 

 pseudopodium, a stalk-like structure which elevates the maturing 

 capsule above the level of the foUage, in very much the same way as 

 does the sporophyte seta in the mosses. In the absence of fertilisation 

 and zygotic development, these several gametophyte developments in 

 liverworts and mosses do not take place. 



anthocerotae 



This group is sometimes included as an order of the Hepaticae, but 

 it has also been treated as a class commensurable with that group and 

 with the Musci. The sporophytic development of Anthoceros is of 

 special interest: it is held by some authors that Anthoceros is the 



