. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. SPORE-PRODUCING MEMBERS 447 fertile condition of the leaf is normally the rule in the family; but that the fertile spike of the Ophioglossaceae behaves like the sporangiophore of the Psilotaeeae, or the sporangium of Isoetes or Lycopodium in the matter of its abortion : this is complete in some leaves, while in others a vestigial structure remains to show what has occurred. Further, though their tendency towards a monophyllous habit may make such a comparison less obvious, the Ophioglossaceae show ess
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. SPORE-PRODUCING MEMBERS 447 fertile condition of the leaf is normally the rule in the family; but that the fertile spike of the Ophioglossaceae behaves like the sporangiophore of the Psilotaeeae, or the sporangium of Isoetes or Lycopodium in the matter of its abortion : this is complete in some leaves, while in others a vestigial structure remains to show what has occurred. Further, though their tendency towards a monophyllous habit may make such a comparison less obvious, the Ophioglossaceae show essentially a " Selago" condition of the shoot, that is, an imperfect differentiation of the vegetative and reproductive functions. Their condition would, in fact, be consistent with a strobiloid origin, modified in further development by enlargement of the appendages, all of which were originally fertile. This matter will be referred to again in connection with the early appearance of the fertile spike in the young seedling plant. Spore-Producing Members. The development of the fertile spike has been traced in Ophioglossum from its first The leaf itself originates very close to the initial cell of the deeply depressed apex of the axis. The sheathing stipule which envelopes the growing point as well as all the later leaves, is formed early: the spike appears above it in a median position on the adaxial face of the leaf, but near to its base (Fig. 245). The outgrowth is at first. Fig. 245. Ophioglossum vulgatum. The lower drawing shows a longitudinal bluntly rounded, but it SOOn becomes more median section of a young leaf, with J . the spike (sj>) arising about half-way acute and turned upwards : it consists of several up its adaxial face. The upper . drawing shows a rather older leaf in cells, and of these the uppermost, which is frontal view. X35. already the largest (x in the Figs. 246 a, b, c, e), Undergoes further segmentation with some degree of regu
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