EMBRYOGENESIS IN THE BRYOPHYTA 77 



of growth along the axis. As is well known, a characteristic intercalary 

 meristem is established at a later stage. In Notothylas orbicularis, as 

 described by Campbell (1918), the first division of the zygote may be 

 at right-angles to the neck of the archegonium, Fig. 17m — the earliest 

 stage observed showed four quadrants of equal size ; but in a species 

 described by Lang (1907) the first wall is as in Anthoceros, Fig. 17h-q. 



MUSCI 



The Musci, comprising the Sphagnales, Andreaeales and Bryales, 

 yield points of interest in the development both of their gametophytes 

 and sporophytes. 



The spore of Sphagnum on germination gives rise to a short filament 

 of three to four cells. Growth is most active in the distal cell. In fact, 

 according to Ruhland (1924), it functions briefly as an apical cell with 

 two cutting faces, a flat thalloid protenema, one cell thick, being formed. 

 On further growth, the apical cell becomes less active, and a protonema 

 of somewhat irregular contour arises by the division of the marginal 

 cells. New protonemal filaments may arise from these marginal 

 meristematic cells and in turn give rise to flat protonemata at their 

 extremity. Later, as a result of active growth and division of a marginal 

 cell, a bud with a three-sided apical cell is initiated and this gives rise 

 to the erect leafy stem. 



The archegonia in Sphagnum are formed at the apex of short 

 branches at the summit of the plant; they are shortly stalked and 

 quite free like those observed in other mosses. The first division of the 

 elongated zygote is by a transverse wall : the polarity is thus determined 

 early, the upper cell becoming the apical cell of the embryo. On further 

 growth the embryo becomes a transversely divided filament, Fig. 18a. 

 The basal cell meanwhile divides by a somewhat oblique longitudinal 

 wall; this is followed by other divisions and a parenchymatous foot 

 with somewhat irregular cellular pattern results. In the epibasal region 

 the apical cell continues to divide by transverse walls and the subjacent 

 cells by longitudinal ones, the resulting cellular pattern affording an 

 almost classic example of Errera's law. Fig, 18b, c. When the embryo is 

 of a cylindrical-conical shape, periclinal divisions take place in the cells 

 of the epibasal region, the amphithecium and endothecium being thereby 

 defined, Fig. 18c, d, f. This may be taken as marking the transition 

 from the embryonic to the sporogenous phase, the further developments 

 consisting in the formation of the capsule and seta from the epibasal 

 segment and of the large distended foot from the hypobasal segment. 

 Fig. 18e. Sphagnum shows an interesting post-fertilisation development 

 in the gametophyte, the distal region enlarging into a stalk-like 

 pseudopodium {see p. 73). 



