Chapter X 

 EMBRYOGENESIS IN LEPTOSPORANGIATE FERNS 



THERE is now a considerable literature on the embryology of lepto- 

 sporangiate ferns. In 1842 Bischoff discovered the embryo enclosed 

 within the tissue of the prothallus, and in 1849 Hofmeister demonstrated 

 that it was formed within an archegonium. In his 'Comparative 

 Investigations' (1851) he gave detailed accounts of the embryogeny in 

 Dryopteris (Aspidium) fiUx-mas and Pteridium (Pteris) aqidlinum. In 

 1863 Pringsheim followed with a study of Sahmia natans and in 1865 

 Hanstein dealt v/ith Marsilea salvatrix. Other investigations appeared 

 from time to time, some relatively complete, others of a more frag- 

 mentary character. Vladesco (1935) has summarised and evaluated the 

 work to date and may be referred to for a comprehensive bibliography. 

 His own work, which includes both morphological and experimental 

 observations, has been drawn upon here for an account of the normal 

 development in leptosporangiate ferns. 



THE EARLY SEGMENTATION AND EMBRYOGENY 



In Dryopteris, Adiautum, Onoclea, Gymuogramme and other 

 leptosporangiate ferns, the archegonia are typically formed on the 

 lower side of the heart-shaped prothallus and have their necks directed 

 downwards, or obliquely downwards. The fertilised ovum is approxi- 

 mately spherical in shape, and the first partition wall, known as the basal 

 wall, lies in the axis of the archegonium and at right-angles to the 

 axis of the prothallus. Figs. 30d, 33-36. By the time this wall has been 

 laid down, the polarity of the embryo has been irreversibly determined : 

 the anterior segment, which lies towards the apex of the prothallus, is 

 the epibasal segment and gives rise to the shoot apex and first leaf; the 

 posterior or hypobasal segment gives rise to the first root and foot. 

 The second wall is at right-angles to the first and it lies in the axis of the 

 archegonium and of the prothallus; it is described as the median wall. 

 The embryo now consists of quadrants which would be seen in plan by 

 looking down on the under surface of the prothallus. The third wall is 

 a transverse one, i.e. it is perpendicular to the archegonial axis and to 

 the first two walls. The embryo now consists of octants. These initial 

 divisions, and those which follow, usually take place in a very regular 

 manner; indeed, they are in close agreement with the ideal segmentation 

 pattern that would be obtained in a sphere dividing by walls of minimal 



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