158 EMBRYOGENESIS IN PLANTS 



APOGAMOUS DEVELOPMENTS 



In many fern species the sporophyte may be formed from the 

 prothallus without the fertilisation of an ovum. The relevant literature 

 has been reviewed by Bower (1923, 1928, 1935), Smith (1938) and 

 Manton (1950). This phenomenon, known as apogamy, has relevance 

 to studies of embryogenesis. Apogamously produced sporophytes may 

 have their inception in a single cell or in a group of cells of the gameto- 

 phyte, in one or more cells adjacent to the base of the archegonium, 

 or in the ovum in an unopened archegonium (parthenogenesis). In the 

 cases of apogamy which have been critically examined cytologically, the 

 prothallial cells are already diploid. However, the apogamous sporo- 

 phyte originates, its organogenic development resembles that of a 

 normal embryo. This can be seen in Pteris cretica, in Nephrodium moUe, 

 or in MarsUea dnimmondii. In each instance the nutrition of the young 

 sporophyte is drawn from the cells of the prothallus. Thus, the young 

 sporophyte, whether produced apogamously or after normal fertilisa- 

 tion, manifests a specific organisation from an early stage. The tenta- 

 tive conclusion may therefore be drawn that the size and the relative 

 development of the organs are determined by genetical factors as the 

 general or pervasive underlying cause, and by the nutritional status of 

 the prothallus, environmental and other factors, as the proximate cause 

 — a view that is justified by such experimental studies as have so far 

 been undertaken. 



Pellaea viridis is well known as a species which typically reproduces 

 apogamously (Steil, 1918; Vladesco, 1935). Antheridia and archegonia 

 are formed in the normal way on the heart-shaped prothallus. There 

 is evidence that the spermatozoids enter the archegonia and that normal 

 fertilisation occurs. An initial, normal zygotic segmentation has been 

 observed; the later segmentation is irregular and the embryo subse- 

 quently aborts. A sporophyte, however, is formed by the active cell 

 division of the prothallial cushion tissue in the vicinity of the archegonia. 

 Whether chemical stimuli from the latter promote the apogamous 

 growth is not known, but active meristematic activity, and later the 

 formation of tracheides, are closely associated with the archegonia. 

 The irregular divisions of the embryo are thought by Vladesco to be due 

 to the pressure of the actively meristematic, adjacent gametophyte tissue. 



Steil (1939) has given a comprehensive review of apogamy, apospory 

 and parthenogenesis in the ferns. With regard to the apogamous 

 development of the young sporophyte, an early and usually unmis- 

 takeable indication is the appearance of tracheides just behind the 

 apical indentation of the prothallus. In different instances, various 

 morphological and histological developments take place close to the 



