80 EMBRYOGENESIS IN PLANTS 



comparable with those described for Andreaea. The formation of 

 obHque walls in both the epibasal and hypobasal cells results in the 

 establishment of two-sided apical cells, on the further growth and 

 division of which a spindle-like embryo is formed, Fig. 18l, m. In 

 some mosses, e.g. Tortula, Fig. IBs, the hypobasal region undergoes 

 little development. When periclinal walls are laid down in the epibasal 

 region, the amphithecium and endothecium are defined. Even in the 

 simpler Bryales, e.g. in cleistocarpic genera such as Archidium and 

 Ephemerum, Fig. 18p-r, the early stages in the embryogeny are as 

 described above. 



Bishop (1929) has recorded the occurrence of well marked fila- 

 mentous embryos in various Bryales, Fig. 18s-v. Early workers, e.g. 

 Hofmeister (1851, 1862), had shown that in mosses such as Fissidens, 

 Phascum and Funaria, the sporophyte is derived almost entirely from 

 the epibasal segment of the two-celled embryo, the hypobasal cell 

 undergoing a few divisions of a somewhat irregular nature. Hofmeister, 

 however, noted that in some species, e.g. Bryum argenteum, the zygote 

 may elongate and the epibasal cell divide by several transverse walls, 

 before the apical cell is established, i.e. the young embryo is essentially 

 filamentous. Bishop has shown that a filament of at least three cells, 

 and usually more, is typically formed before the two-sided apical cell 

 becomes functional in Mnium honmm. Fig. 18t, v, M. punctatwn and 

 Bryum capillare. A filament of nine cells was observed in M. hormmu 

 this being the result of the formation of transverse walls acropetally in 

 the epibasal segment. A seven-celled filamentous embryo was found 

 to be typical of Catharuwa undidata. Fig. 18u, and it may well be that 

 filamentous stages will be found in many species. Bishop has also 

 illustrated a remarkable elongation of the fertihsed egg in these three 

 genera and calls attention to the long, narrow archegonial pedicel or 

 stalk in them. Figs. 18u, v. The occurrence of these elongated filamen- 

 tous embryos, and their characteristic mode of division, lend strong 

 support to the view that acropetal gradients are important factors in 

 the embryogenic development. 



DISCUSSION 



The bryophyte sporophyte is small and limited in its organographic 

 development, the sporogenous phase almost immediately supervening 

 upon the brief and simple embryogeny. Each species, however, is 

 characterised by the distinctive size, highly regular development and 

 organisation of its sporophyte. 



If we regard the embryonic development as ending with the 

 periclinal divisions that lead to the initiation of archesporial tissue, then 

 the task of the embryologist is to attempt to explain such phenomena 



