Chapter V 

 EMBRYOGENESIS IN THE BRYOPHYTA 



THIS subdivision includes the simplest and most primitive members 

 of the Embryophyta, i.e. land plants in which an embryonic phase 

 follows the fertilisation of an enclosed ovum. Bryophytes and pteri- 

 dophytes have been treated by some botanists as subdivisions of one 

 great and related group — the Archegoniatae; but others have held 

 that the presence of an archegonium in the two groups is merely 

 evidence of parallel evolution from algal ancestors. In bryophytes the 

 conspicuous phase of the life cycle is the gametophyte, the plant 

 consisting of a flattened thallus in liverworts such as Marchautia or 

 Pellia, or of a leafy or foliose axis in the leafy liverworts and mosses. 

 Although some of these gametophytes are at a relatively low level of 

 organisation, their development can be referred to an apical growing 

 point : hence it may be of interest to consider the development both of 

 the free haploid spore and of the enclosed zygote. 



The embryonic development, which follows the fertilisation of the 

 ovum, takes place within the enlarging venter of the archegonium. 

 Indeed, in many bryophytes, the sporophyte remains within an 

 enveloping calyptra — an organ of gametophyte origin — until its 

 dissolution at spore dispersal. The whole sporophytic development in 

 bryophytes is of a very limited kind — a brief embryogeny on which a 

 sporogenous phase quickly supervenes. The initial embryonic phase, 

 in fact, passes almost directly into the sporogenous, reproductive phase, 

 whereas, in other classes of Embryophyta, the embryonic and reproduc- 

 tive phases are separated by a very considerable period of vegetative 

 development. 



In the liverworts the sporophyte remains dependent on the game- 

 tophyte for most of its nutrition and for protection until the time of 

 spore dispersal {see pp. 82, 83). In the mosses, with some exceptions, 

 the sporophyte attains to a greater somatic differentiation and develop- 

 ment, and contributes to its nutrition by means of a photosynthetic 

 system. The several aspects of vegetative development in the bryophytes 

 were held by Bower (1908, 1935) to exemplify the advance of the 

 sporophyte, but others, e.g. Wettstein (1903-1908), Goebel, (1930), 

 Evans (1939) and Bold (1948), have given reasons for the view that the 

 Hepaticae would more truly be interpreted as exemplifying a reduction 

 series {see p. 83). In phanerogams the embryo in the mature seed 



63 



