EMBRYOGENESIS IN EUSPOR ANGI ATE FERNS 131 



determined at a very early stage, and the foot and first root, as also the 

 first root and first leaf, are invariably contiguous. The first leaf, like 

 the second and all subsequently formed leaves, is a dorsiventral 

 structure, and arises in the normal foliar orientation to the radially- 

 symmetrical shoot apex. It has been shown experimentally that not 

 only do the leaves of ferns and flowering plants always originate in the 

 shoot apical meristem, but that their characteristic dorsiventrality is 

 probably due to biochemical effects proceeding from the shoot apical- 

 cell-group. (Wardlaw, 1952; Sussex, 1951, 1952). All this is as much 

 as to say that in O. vu/gatum, no matter how sluggish and seemingly 

 anomalous the embryonic development may be, a characteristic bio- 

 chemical organisation, or pattern, not unlike that of other ferns, is 

 present from an early stage and is maintained and elaborated during 

 the subsequent development. (For further discussion of this aspect 

 see p. 165.) 



As a working hypothesis we may suppose that, for the 'normal' 

 development of a fern embryo, each quadrant requires particular meta- 

 bolites in certain characteristic concentrations and proportions. Or, 

 alternatively, we might say that each quadrant develops as it does 

 because it is 'normally' supplied with certain metabolites in charac- 

 teristic concentrations and proportions. Now whereas the 'normal' 

 embryo, as seen in Dryopteris, Adiantum or Osmunda, derives its 

 nutrition from a green, photosynthetic prothallus, the embryo of 

 O. vulgatum is nourished by a saprophytic, mycorrhizic underground 

 prothallus. The Dryopteris type of embryo {see below, p. 142) is 

 characterised by rapid growth, by the early and approximately simul- 

 taneous formation of the apices of shoot, leaf and root, and by a 

 balanced embryonic development. By contrast, the embryo of O. 

 vulgatum is characterised by very slow growth, by heavy depositions of 

 starch in the embryonic cells, by relatively precocious root formation 

 and very belated shoot and leaf formation. However, once a photo- 

 synthetically active leaf has appeared above-ground, the organogenic 

 development becomes 'normal' and 'balanced' and, indeed, in all its 

 general features, is closely comparable with that of Dryopteris. The 

 delayed organisation of the shoot apex in O. vulgatum has a close 

 parallel in those species of Lycopodium where the young embryo also 

 derives the whole of its nutrition from a mycorrhizic, underground 

 prothallus {see Chapter VII). A reasonable iniference from the facts is 

 that whereas the mycorrhizic fungus provides the prothallus (and, 

 indirectly, the young embryo) with an abundance of carbohydrate, the 

 supply of nitrogen-containing substances, and possibly of certain 

 growth-regulating substances, is small and limiting : hence the delay in 

 the formation of the protein-synthesising shoot apical meristem. 



