Chapter VI 

 EMBRYOGENESIS IN THE PSILOTALES AND EQUISETALES 



PSILOTALES 



T^siLOTUM and Tmesipteris, the only two genera in this pteridophyte 

 1 order, illustrate the embryonic development in the most primitive 

 of living vascular plants. Both genera are rootless and Psilotum is also 

 leafless. Their closest affinity is with the ancient Palaeozoic fossil 

 group, the Psilophytales, which includes such leafless and rootless 

 genera as PsiJophyton, Rhynia and Horneophyton. Nothing is known of 

 embryonic development in the fossil forms. 



In both Psilotum and Tmesipteris, Figs. 19, 20, the prothalli are 

 branching, irregularly cylindrical, mycorrhizic bodies, devoid of 

 chlorophyll and completely saprophytic. The short-necked arche- 

 gonium is immersed in, and in cellular continuity with, the prothallial 

 tissue. The early embryogeny is similar in the tv/o genera. After 

 fertilisation, the ellipsoidal zygote enlarges, fills the venter and divides 

 by a transverse wall. The polarity of the embryo is now defined. The 

 embryogeny is exoscopic and in due course the epibasal segment gives 

 rise to the shoot apex and shoot, and the hypobasal segment to a 

 parenchymatous haustorial foot. The first divisions of both the 

 epibasal and hypobasal cells are by walls lying in the axis of the 

 archegonium. On further growth, transverse walls are laid down. 

 Thereafter the histological development of the foot is somewhat 

 irregular; it becomes roughly cylindrical in shape and grows down into 

 the gametophyte tissue from which it withdraws the nutrition required 

 by the embryo by means of short haustorial cells. In this connection, 

 comparisons with the haustorial foot in Anthoceros, Fig. 17, have been 

 made. After the quadrant stage the divisions in the epibasal segment 

 tend to be somewhat irregular, but soon a single initial or apical cell 

 can be distinguished, usually in an approximately median position. 

 The further growth of the shoot is due to the activity of this cell. In 

 some quite young embryos, two separate apical initials are established 

 on opposite sides of the rounded distal region, their further develop- 

 ment resulting in the formation of a vasculated, dichotomously 

 branched embryo, Fig. 19d, g. As these developments are accompanied 

 by growth of the adjacent gametophyte tissue, the embryo becomes 

 overarched by a calyptra-like structure. On emerging from the calyptra, 

 the embryo becomes infected by the mycorrhizic fungus and develops 



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