EMBRYOGENESIS IN LEPTOSPOR ANGIATE FERNS 149 



(i) the existence of a dislinclivc biochemical pattern in the spherical 

 embryo at, or soon after, the octant stage, (ii) the organisation of the 

 shoot apical meristem as a morphogcnetic region, (iii) the functioning 

 of the apical cell (or apical cell group) as a growth centre whose physio- 

 logical field affects the growth of the adaxial side of the leaf, and (iv) 

 the effect of light, gravity and polarity in effecting characteristic 

 accumulations of auxin in the leaf and root, i.e. the anterior- and 

 posterior-inferior quadrants, may contribute to an understanding of 

 the observed developments. 



In the leptosporangiate ferns, as in other pteridophytes, the epibasal 

 segment of the zygote, or distal pole, soon becomes a self-determining 

 morphogcnetic region, drawing nutrients to itself from the foot. It 

 seems not unlikely that auxin, produced during the growth of the 

 incipient shoot and leaf apices, diffuses backwards and is differentially 

 distributed in the two hypobasal quadrants, promoting root formation 

 and growth in one quadrant and parenchymatisation in the other. 



In the young embryo, as in the adult fern, the leaves grow much more 

 rapidly than the shoot. Nevertheless, in leptosporangiate ferns, the 

 sporophyte may well be considered to be an axial structure from the 

 outset. Apart from the first leaf, v/hich originates simultaneously with 

 the shoot, the shoot apex gives rise to the leaves and not vice versa. 



BRIEF SURVEY OF LEPTOSPORANGIATE FERNS 



Bower (1926) has placed the Osmundaceae in a central place am.ong 

 the Eusporangiatae, but notes that they show certain evolutionary 

 upward tendencies: they are the contemporary representatives of an 

 ancient phyletic line, somewhat intermediate in morphology between the 

 eusporangiate and leptosporangiate ferns. Campbell (1895, 1918, 1940) 

 has maintained that they belong to the Leptosporangiatae. The 

 archegonium and embryogeny are like those of this subdivision, 

 though there are some differences worthy of note. 



In Osmimda an octant stage of some regularity has been observed 

 (Campbell, 1918, 1940), Fig. 33. The first two walls are both in the 

 axis of the archegonium; the third wall is transverse; and the inception 

 of the primary organs from the quadrants and the post-octant seg- 

 mentation pattern are as in the Polypodiaceae. The post-octant 

 segmentation pattern, however, is considerably less regular, and 

 comparative morphologists have regarded this as evidence of the 

 essentially intermediate position between eusporangiate and lepto- 

 sporangiate ferns occupied by the Osmundaceae. The Osmunda embryo, 

 moreover, retains its globular form for a longer time, i.e. there is a 

 delay in the inception of its primary organs, Fig. 33c, d, and it does not 

 emerge from the calyptra until it has attained to a considerable size, 



