EMBRYOGENESIS IN LE PTOSPOR ANGI ATE FERNS 155 



Drvopteris parasitica is generally like D.filix-mas. Vladesco (1931) 

 has figured an interesting case of two embryos developing within the 

 same archegonium. 



In Scolopemiriwn nilgare two or three sporophytes may be found 

 on vigorous individual prothalli (Vladesco, 1935). The zygote is 

 spherical but sometimes elongated along the axis of the prothallus. 

 The post-octant divisions in the epibasal segments are somewhat 

 different from those found in other leptosporangiate ferns. The partition 

 walls run from the basal wall to the curved outer surface, and no 

 'epibasal disc' is therefore formed, Fig. 34. In other Polypodiaceae, 

 such as Athvrium fHix-foetnina, Bleclmum orientale, Woodsia ilvensis, 

 etc., the details of the embryonic development are typically those of 

 leptosporangiate ferns (Vladesco, 1935). The same is true of such 

 Cyatheaceae, e.g. Alsophiki australis and Cibotium sp., as have been 

 investigated and of Ceratopteris thalictroides (Parkeriaceae). 



In the heterosporous order, the Marsileaceae, comprising Marsilea 

 and Pilularia, the large oval megaspores undergo development on being 

 shed from the sporocarp. The apical region of the megaspore becomes 

 organised as a reduced prothallus with, usually, a single projecting, 

 short-necked archegonium, Fig. 36. After fertilisation, the embryonic 

 development takes place very quickly— in M. vestita the first wall 

 appears within one hour of fertilisation and the first leaf can be seen 

 within 24 hours — and is closely comparable with that of homosporous 

 leptosporangiate ferns. As the embryo develops, the cells of the 

 surrounding venter are stimulated to grow and divide by periclinal 

 walls, the enlarging venter thus forming a two-layered calyptra which 

 continues to ensheath the embryo until the formation of the root and 

 first leaf is well advanced. Campbell (1940) states that the limits of the 

 root and foot are not clearly defined and indicates the probability that 

 the whole embryonic region contiguous with the food resources in the 

 prothallus 'no doubt functions as a haustorium.' Thomson (1934) has 

 shown experimentally that the form of the haustorial foot can be 

 modified by excising the young embryo. 



The first leaf in Pilularia and Marsilea^ is non-laminate or awl- 

 shaped, but the later-formed leaves of Marsilea are bifid and tetrafid ; 

 in Pilularia all the leaves are awl-shaped; in Regnellidium the adult 

 leaves are typically bifid. The further development of the embryo of 

 Marsilea under controlled conditions has been studied in some detail by 

 Allsopp (1951-1953). 



In the other order of heterosporous ferns, the Salviniaceae, including 

 Salviuia and Azolla, the archegonium in the latter is formed in the 

 small prothallus which occupies the apical end of the megaspore. The 



^ In some species of Marsilea the first leaf is somewhat spatulate. 



