160 EMBRYOGENESIS IN PLANTS 



fertilised archegonia were variously orientated and rotated, Leitgeb 

 (1880) found that the positions of the basal wall and of the several 

 nascent organs were not altered. The action of light and of the sub- 

 strate was also ruled out; in short, external factors were apparently 

 not primarily involved. Heinricher (1888) illuminated the dorsiventral 

 prothalli of Ceratopteris firstly from above, so as to induce archegonia 

 on the lower or shaded side, and then from the lower side so that 

 archegonia were induced on the upper side. As Fig. 30e shows 

 diagrammatically, the leaf and root quadrants are directed upwards in 

 the left-hand archegonium, in which light and gravity are acting in 

 opposite directions, and downwards in the right-hand archegonium 

 where the directions of light and gravity coincide. In both archegonia 

 the root and leaf quadrants are adjacent to the neck; also, in both, the 

 epibasal segment, which comprises the shoot and leaf quadrants, is 

 directed towards the apex of the prothallus. These experimental results 

 seem to show that external factors such as light and gravity have little 

 or no effect on the differentiation of the embryo. Wetter (1952) 

 considers that this finding for Ceratopteris probably holds good for all 

 leptosporangiate ferns. All the attendant circumstances support the 

 conclusion, with which the present writer is in accord, that the polarity 

 of the zygote is determined by gradients in the parent prothallial tissue, 

 the position of the first nuclear spindle and of the basal wall being 

 evidence of that polarity and not the cause of it (A. Lang, 1949; 

 Wetter, 1952). It is not impossible that auxin or some other growth- 

 regulating substance, formed at the prothallus apex and moving 

 basipetally, as demonstrated by Albaum (1938), is the primary deter- 

 miner of polarity. Ahernatively, both acropetal and basipetal metabolic 

 gradients in the prothallus may determine not only polarity, but also 

 the biochemical pattern in the egg or in the zygote. Wetter calls atten- 

 tion to the fact that the orientation of apogamous sporophytes is 

 essentially the same as that of the normal embryo. 



The Effect of Excising the Embryo. Ward (1950) has shown by the 

 use of a surgical technique that the embryonic development of PoJy- 

 podium aureum can be modified. By incising the prothallial tissue close 

 to a fertilised archegonium, he effected the partial release of the young 

 embryo. This embryo outgrew the calyptra much earlier than did a 

 normal embryo of comparable size. The treated embryo developed as 

 a slow-growing embryonic mass, at first lacking appendages and with- 

 out definite shape, but later it differentiated a stem and leaf as in the 

 normal embryogeny. There was, however, no root development. The 

 extreme effect observed in these released embryos was the production 

 of an enlarged slow-growing tuberous mass of parenchymatous tissue 

 of indefinite form and lacking vascular tissue. Several such masses 



