130 EMBRYOGENESIS IN PLANTS 



and emerges above ground as the first green leaf, this process being said 

 to require some eight to ten years. This leaf is of small size and bears 

 no fertile spike. The third leaf, which appears above-ground the follow- 

 ing year, may, in some instances, be fertile. The further developments 

 are as in the mature plant. Young sporophyte plants, both free in the 

 soil or still attached to the prothallus, may have one to four well 

 developed, horizontally disposed roots, while the shoot apex is still a 

 very minute bud. In one specimen figured by Bruchmann, a secondary 

 adventitious bud is present on a long root, the bud being at some 

 distance away from the primary apex. Like the parent prothalli, all 

 these plantlings occur several centimetres below ground and are 

 holosaprophytic. 



In terms of growth the following points emerge from a consideration 

 of the embryological data given above : (i) the growth of the embryo 

 takes place very slowly until a green leaf has appeared above-ground, 

 i.e. mycorrhizic nutrition alone apparently admits of very slow growth 

 only; (ii) the cells of the prothallus and young embryo are densely 

 packed with starch grains; (iii) the nutrition of the embryo from the 

 prothallus may at first be by v/ay of the foot and root but subsequently, 

 when the root has burst out of the prothallus, the foot must be the 

 principal organ of uptake; (iv) root inception and development take 

 place in advance of the organisation of the shoot apex and the formation 

 of the first leaf; (v) as judged by the inception of vascular tissue, the 

 shoot apex is relatively inert, the vascular tissue being largely, if not 

 completely, foliar in origin. 



Although the times of development of the different parts of the 

 young embryo (shoot, leaf, root and foot) in the eusporangiate fern 

 O. vulgatum are difterent from those in leptosporangiate ferns such as 

 Dryopteris or Adiantum, the eventual patterns of embryonic develop- 

 ment are closely comparable in the two types. Thus the embryo in 

 Adiantum, Fig. 35, shows a characteristic spherical quadrant stage, after 

 which the foot and root are formed from the two hypobasal quadrants 

 and the shoot and first leaf from the epibasal quadrants; also the leaf 

 and root quadrants are contiguous. This is just the relationship that 

 has been described above for Ophioglossum vulgatum. It is, therefore, 

 a reasonable assumption that the same general factors are at work in 

 both ferns, and that constitutionally they have much in common. 



Although in Ophioglossum vulgatum the embryonic development, 

 and especially the organisation of a shoot apex, take place very slowly, 

 it seems probable that the essential and primary differentiation of the 

 several embryonic regions is a biochemical one, i.e. it is due to the 

 inception of a patternised distribution of particular morphogenetic 

 substances. The polarity of the embryo, as in other embryogenies, is 



