EMBRYOCiENESIS IN LEPTOSPOR ANGI ATE FERNS 161 



eventually gave rise to three plants with apparently normal stems and 

 leaves. Other curious developments were also noted. Embryos 

 isolated from the apex of the prothallus were notably slow in develop- 

 ment as compared with those isolated in other ways, this being 

 attributed to the interruption of the basipetal auxin gradient. Ward 

 considers that the morphological developments which ensued upon the 

 artificial release of the young embryo from the restraint of the prothallial 

 tissue indicate that formative effects are normally exerted on the 

 developing embryo by the archegonium and derived calyptra. 



As already mentioned, Thomson (1934) showed that when young 

 embryos of Marsilea were excised and grown in culture solution changes 

 could be observed in the shape of the foot. 



The Segmentation Pattern. The regular system of cell cleavages has 

 been attributed by various investigators to the changing energy 

 relationships in enlarging cells and to the action of surface forces. So 

 far as the writer is aware, these ideas have not been tested experimentally 

 in the ferns. 



Nutritional Factors and Embryonic Developments. Practically 

 nothing is known of the efifect of nutritional factors on the early 

 embryonic development in leptosporangiate ferns: the relationship 

 between the zygote and the adjacent cells of the prothallus is evidently 

 a very intimate one. 



Klebs (1916) showed that the morphology of the prothallus can be 

 varied according to the composition of the culture solution and the 

 incidence of the factors of light and temperature (for a review of this 

 aspect, see Williams, 1938). Becquerel (1931) tried to use the method 

 of pure culture of prothalli in mineral solutions to study the early 

 embryogeny. Vladesco (1935) used similar methods and found that 

 in Gymnogramme sulphurea the first divisions of the zygote were closely 

 comparable whether the prothallus bearing it was growing on moist 

 burnt soil, floating on a culture solution, or submerged in it. In some 

 submerged embryos, the epibasal segment appeared to be larger than 

 the hypobasal segment, this being a reversal of the usual condition in 

 leptosporangiate ferns. Moreover, whereas the subsequent divisions 

 took place normally in the epibasal segment, development was dela3'ed 

 in the hypobasal segment and the orientation of the partition walls was 

 atypical. These observations would appear to indicate that the bio- 

 chemical pattern of the zygote had been somewhat modified by its 

 development under submerged conditions (probably diminished supply 

 of oxygen and light intensity). Vladesco also records that a root initial 

 was very rarely differentiated in submerged embryos, the plantling 

 being rootless or with a small abortive root. By contrast, it was rare 

 to find that a root failed to develop, or became abortive, in prothalli 



