EMBRYOGENESIS IN THE LYCOPODINEAE 117 



hypobasal tier gives rise to the foot only, while the epibasal tier forms 

 the first leaf and root and, after some delay, the shoot apex, Fig. 29d. 

 In the further development from the octant stage, the embryo consists 

 of an ovoid mass of similar cells. It then widens out in the plane of the 

 basal wall : one half of the epibasal region gives rise to the first leaf 

 and the other, opposite half to the first root, Fig. 29d. A large cell at 

 the base of the first leaf is recognised as the primordium of the ligule. 

 The differentiation of the ligule is followed by the formation of a semi- 

 circular ridge at the base of the root; this grows rapidly as a sheathing 

 organ or velum and encloses the base of the leaf and its ligule. In the 

 narrow cleft between the bases of the velum and first leaf there is an 

 inconspicuous group of cells. These, in fact, are meristematic cells of the 

 small and as yet unorganised shoot apex, of which, according to 

 Campbell (1918), there is no indication at earlier stages in the embryo- 

 geny. Even for some time after, the shoot apex does not show any 

 conspicuous organisation: there is general agreement among in- 

 vestigators (Hegelmaier, 1872, 1874: Farmer, 1891; Lang, 1915) that, 

 as in Lxcopodium and Selagiuella spimdosa, no single initial cell can be 

 observed at the apex. Campbell (1918) regards the first leaf ('cotyledon') 

 with its ligule as the first organogenic development, this being followed 

 by the inception of the first root opposite to it. Between these two 

 organs, in a slight depression, is the locus of the shoot apex, which is 

 still very small, feebly developed and quite unorganised. The second 

 leaf is formed facing the first, with the inconspicuous shoot apex lying 

 between. The subsequently formed leaves arise in a spiral sequence 

 round the apex. 



Campbell has noted that while the axes of the first leaf and first 

 root may, or may not, coincide, their vascular strands are continuous. 

 The foot is poorly developed in the young embryo, but becomes quite 

 conspicuous later on. Fig. 29d, gradually filling the megaspore cavity. 

 At this stage the prothallial tissue has virtually disappeared. 



Many, probably most, investigators have regarded Isoetes as a 

 ligulate lycopod, admittedly presenting many unusual features but, 

 on the concensus of evidence, being such as to be included in the 

 Lycopodineae and to be brought into a general, if not close, relationship 

 with Selagiuella. This, for example, is the view taken by Bower. But 

 Campbell (1940), while agreeing that the ligulate leaf, the presence of 

 microspores and megaspores, and the nature of the prothallus admit 

 of fairly close comparisons being made between Isoetes and Selagiuella, 

 considers that Isoetes differs so much from other pteridophytes that its 

 true affinity must remain a matter for conjecture. On balance, he 

 considers that it may have a closer affinity with eusporangiate ferns, 

 such as the Marattiales and Ophioglossales, than with Selagiuella. 



