14 



EMBRYOGENESIS IN PLANTS 



ferns, and in some bryophytes, e.g. Anthoceros, the first wall lies in the 

 axis of the archegonium, Fig. 2. These facts suggest that polarity is not 

 determined by structural features of the archegonium itself. In the 

 encapsulated embryo in bryophytes and vascular plants, the establish- 

 ment of polarity brings the embryo into a particular position in relation 

 to the gametophyte plant (e.g. the moss plant or the pteridophyte 



Fig. 2. The first division of the zygote or spore in different groups 



A, Spirogyra velata, germinating zygospore (after West and Fritsch). B, Fitcus sp. 

 (after Rostafinski). C, Polysiphonia alrorubescens, germinating tetraspore (after 

 Chemin). D, Targionia hypophylla (after Campbell). E, Radiila sp. (after Leitgeb). 

 F, Osmuuda claytoniaiia, germinating spore (after Campbell). G, AdiaiUiim concin- 

 num, divided zygote (after Atkinson). H, Lycopodium phlegmaria (after Treub). 

 J, Clienopodium bonus-hemiciis. K, Liiiida forsteri (after Soueges). 



prothallus) or gametophyte tissue (in seed plants). As this early 

 establishment of a polarised axis has many consequences in the sub- 

 sequent embryogenic development, it is evident how essential it is to 

 have some knowledge of the factors which may bring it about. 



As early as 1878 Vochting, by dissecting plants into smaller and 

 smaller segments, was able to demonstrate that every fragment possessed 

 definite polarity (Vochting, 1878, 1884). As polarity was evident in 

 even the smallest fragment this phenomenon would, by implication, 

 extend to the individual cell. However, as later workers (Gocbel, 1908; 

 Schwaritz, 1935) were to show, the polarity and morphogenetic be- 

 haviour manifested by single cells or groups of cells depend on the 

 fragment as a whole. 



