126 EMBRYOGENESIS IN PLANTS 



Although the prothalli of many ferns are very much alike, i.e. 

 consisting of a thin strap-like or heart-shaped sheet of photosynthetic 

 tissue, there is, nevertheless, some diversity in the morphology of the 

 prothallus. Thus, some are underground and mycorrhizic and entirely 

 saprophytic in their nutrition. As the embryo and young sporophyte 

 are initially dependent on the prothallus, there is a question as to what 

 extent, and in what way, the embryonic development is determined, or 

 modified, by nutritional factors. 



The types of zygotic segmentation in the ferns are illustrated in 

 Fig. 30. 



OPHIOGLOSSACEAE 



The Ophioglossaceae, including the living genera OphiogJossum, 

 Botrychium and Helmiuthostachys, are probably the most primitive of 

 living ferns and may well be regarded as modern survivors of ancient 

 Palaeozoic stocks. Actually no fossil progenitors are known. The 

 Ophioglossaceae afford very typical examples of eusporangiate ferns 

 (Bower, 1926, 1935; Campbell, 1911, 1918, 1940). 



In all the Ophioglossaceae the gametophyte is a tuberous, mycorrhi- 

 zic, saprophytic, underground body. The prothalli of Ophioglossum 

 and HelminthostacUys are typically elongated, cylindrical structures, 

 sometimes branching, with a distal apical growing point; that of 

 Botrychium is a dorsiventral, somewhat flattened structure, but growth 

 is also by means of a meristematic apex. The prothalU thus afford 

 evidence of features which we normally associate with the embryo. 



OPHIOGLOSSUM VULGATUM 



As only some of the early stages in the embryogeny of O. vulgatum 

 have been ascertained, the relation of the organs to the initial divisions 

 of the zygote still await clarification, Bruchmann's valuable paper 

 (1904) does, however, give a substantial amount of information. The 

 archegonia are situated at the surface of the cylindrical prothallus. 

 The first division of the zygote is by a transverse wall, Fig. 31a. No 

 suspensor is formed and the embryogeny is exoscopic. The segment 

 next to the archegonial neck may be recognised as the epibasal segment 

 and the other as the hypobasal segment. The quadrant and octant 

 stages, though not yet observed, probably exist, for Bruchmann's next 

 illustration is of a median section of an ellipsoidal embryo, in which the 

 octant walls and those subsequently formed are indicated. Fig. 31b. 

 In one hypobasal quadrant the inception of an endogenous root is 

 indicated; the other hypobasal quadrant becomes the foot; but, for 

 some considerable time, the whole of the epibasal region remains in the 

 initial undifferentiated condition, all the cells being meristematic and 



