138 EMBRYOGENESIS IN PLANTS 



we can see that in different species there are factors which have an 

 effect on the initial growth and division of the zygote. These several 

 specific developments afford evidence of differences in the biochemical 

 pattern and reaction in the ovum or zygote. It may be that, within the 

 genus, comparatively small differences in the hereditary constitution 

 may account for the presence or absence of a suspensor. 



HELMINTHOSTACHYS ZEYLANICA 



While the' early stages in the embryogeny of this species are im- 

 perfectly known, it has been demonstrated that a suspensor is present 

 (Lang, 1902, 1910, 1914). The cylindrical prothallus grows vertically 

 in the soil, the archegonia and later the young embryo being aligned in 

 an approximately horizontal direction. The zygote elongates in the 

 axis of the archegonium and extends into the prothallial tissue before 

 any segmentation talces place. The first two walls are transverse: the 

 small, innermost segment is the embryonic cell; the two larger cells 

 constitute the suspensor. Fig. 31j-o. The outer suspensor cell may 

 remain undivided or it may divide into a number of smaller cells. The 

 second tier of the suspensor divides into several cells. The essentially 

 axial embryo continues to grow straight for some time, the embryonic 

 cell giving rise to a hypobasal segment, which develops into the foot, 

 and an epibasal segment which gives rise to the shoot apex, the first 

 leaf and probably the first root. The apex has its inception near the 

 centre of the epibasal tier. As the growing embryo responds to the 

 stimulus of gravity its axis becomes curved and the shoot assumes an 

 erect position. At this stage the embryo is compact and fleshy and is 

 reminiscent of the embryo in Botrychium. Later, the embryo consists 

 of a somewhat elongated hypocotyl terminated by the shoot apex and 

 a first, rudimentary leaf, a conspicuous fleshy foot embedded in the 

 prothallus, and a first root. Fig. 31l. The second leaf is functional and 

 according to Campbell (1940) resembles the first leaf in B. obliquum. 

 He also considers that in Hehninthostachys, in relation to 'the greater 

 development of the axis and terminal bud,' the vascular strand which 

 runs from below the apex to the root tip may be partly of axial or 

 cauline origin. The shoot meristem has a truncated apical cell like that 

 in OphiogJossum. The axis of the young sporophyte is at first upright 

 but eventually becomes a horizontal creeping rhizome. 



In discussing the embryology in the Ophioglossaceae as 'a very 

 interesting problem in morphology,' Bower (1935) considers that the 

 facts support the view that the presence of a suspensor is evidence of an 

 archaic condition and that 'by its elimination the embryo has solved an 

 awkward problem of orientation. So long as the suspensor anchors the 

 embryo in relation to the archegonial neck, contortion of the sporeling 



