64 EMBRYOGENESIS IN PLANTS 



constitutes a well-defined 'stage' in the development of the sporophyte : 

 in bryophytes, no such definitive embryonic stage can be indicated. 

 The growth of the sporophyte is continuous and uninterrupted and the 

 embryonic and adult phases are, as it were, telescoped together so that 

 it is virtually impossible to say where one ends and the other begins. 



EARLY DEVELOPMENT OF THE GAMETOPHYTE 



The haploid spore on germination gives rise to a germ-tube or 

 filament, sometimes described as a protonema. From the outset, as in 

 Riccia, there is an evident movement of the granular cytoplasmic 

 materials from the spore to the distal end of the germ-tube, a densely 

 protoplasmic growing region being thus established. This filamentous 

 development is generally comparable with that seen in the algae and 

 with the initial embryonic development in vascular plants. The distal 

 region of the filament next becomes separated off from the remainder 

 by a transverse wall. As growth continues, further transverse and then 

 longitudinal walls are laid down. The distal cell retains its identity as a 

 recognisable apical cell, the eventual multicellular thallus being formed 

 by its continued meristematic activity. In diff'erent species characteristic 

 and regular segmentation patterns can be recognised both in the initial 

 developments and at the apical meristem, Figs. 6h-l; 15m; 17l. In 

 some bryophytes, e.g. Pellia, Fig. 15m, and Sphagnum, the spore 

 contents may undergo a number of regular divisions, a cellular pattern 

 being thereby constituted before the germling emerges from the 

 exospore. The moss spore on germination yields a branching fila- 

 mentous structure — the protonema — which grows by the division of a 

 distal cell. This initial development closely resembles that of fila- 

 mentous green algae. Thus, although we are here dealing with game- 

 tophytic material, i.e. with the growth of a spore which is not encased 

 in surrounding tissue, nevertheless the early stages of development are 

 not unlike some of those encountered in the development of the 

 sporophyte. In particular, we may note that although the liverwort 

 thallus, e.g. that of Riccia or Marchantia, is a non-vascular structure, 

 it nevertheless attains to a certain level of organisation ; and this can 

 be referred to the early establishment and subsequent activity of an 

 apical growing point. In some Jungermanniales, and in the mosses, 

 the gametophyte is an axial structure consisting of an axis and lateral 

 foliar members. Moreover, in large species such as Polytrichum, a 

 primitive or incipient central conducting system is differentiated. All 

 these features, both external and internal, are due to the activity of an 

 apical cell. Here, then, we have gametophytic developments which are 

 in some important respects comparable with the sporophytic develop- 

 ments found in the pteridophytes and seed plants. 



