EMBRYOGENESIS IN LE PTOS POR ANGI ATE FERNS 159 



prothallus indentation, e.g. a tongue-like or cylindrical outgrowth may 

 appear, or a group of prolliallial cells may become pale in colour and 

 tracheides are dilTerentiated within. The very young apogamous 

 embryo, i.e. one in which the first leaf has not yet appeared, is typically 

 surrounded by multicellular hairs and sometimes by scales; it forms a 

 shoot, leaf and root but not a foot; but in all other respects it closely 

 resembles the embryo produced as a result of fertilisation. In short, 

 the development of an apogamous embryo is very much like that of a 

 sporophyte bud. 



EXPERIMENTAL INVESTIGATIONS 



Thus far the experimental investigation of leptosporangiate fern 

 embryos is not in any way commensurate with the importance of the 

 subject, but a beginning has been made. There are, of course, serious 

 technical difficulties but they should not be regarded as insuperable. 

 Experimental fern embryology, indeed, affords a challenging but 

 relatively unworked field, and seems likely to yield important results to 

 the investigator who can bring new ideas and new techniques to bear 

 on it. 



The following aspects have been investigated by experimental 

 methods : (i) the effect of gravity, light and other physical factors on 

 the early segmentation and disposition of the embryo; (ii) the effect 

 of excising the embryo; (iii) the effect of nutritional factors; and (iv) 

 regeneration. 



The Ejfect of Physical Factors. Sadebeck (1879) held the view that 

 the orientation of the first partition wall in the zygote was determined 

 by factors in the external environment, notably by gravity. In attempts 

 to ascertain the effect of gravity on embryonic development, Leitgeb 

 (1878) used the megaspores of Marsilea, which are large enough to be 

 picked up and placed in different positions, rotated on a kleinostat, and 

 so on. He showed that the first wall could be induced to occupy 

 different planes but these were always about the axis of the archegonium, 

 the wall being horizontally disposed. As a result the epibasal segment 

 was directed upwards. The first wall, however, could not be induced to 

 form in any plane other than that of the archegonial axis. Thus, while 

 gravity exercises some effect in this species, factors intrinsic to the 

 zygote-archegonium complex appear to be the master factors in the 

 embryonic development. Some of the earlier studies of this aspect have 

 recently been reviewed by Wetter (1952). 



Several investigators have shown that the dorsiventrality of the 

 fern prothallus is essentially a light-induced effect and the same factor 

 apparently determines the position and orientation of the archegonium. 

 In experiments with Ceratopteris thalictroides, in which prothalli v/ith 



