SPHENOPSIDA 113 



cells in four rows. There may be a single neck canal cell or 

 there may be two boot-shaped cells, lying side by side, as 

 illustrated in Fig. 16R. There is also a ventral canal cell. 

 The antheridia are sunken in the tissue of the basal cushion, 

 but may also occur on the aerial lobes. They are massive and 

 give rise to large numbers of antherozoids, which are spirally 

 coiled and multiflagellate (Fig. 16P). 



The first division of the zygote is in a plane more or less 

 at right angles to the axis of the archegonium. No suspensor 

 is formed and the embryo is exoscopic. Fig. 16S shows the 

 spatial relationships of the stem apex (x), the first leaves (1), 

 the root (r) and the foot (f), as described as long ago as 1878, 

 but it is now becoming clear that the various parts of the 

 embryo are not so constant in position and origin as was 

 formerly thought. 



There can be little doubt that the Equisetales are related 

 to the Calamitales, but it is most unhkely that they represent 

 their direct descendants. Remains of herbaceous plants 

 resembHng Equisetum are placed in the genus Equisetites, 

 They are traceable right back through the Mesozoic to the 

 Palaeozoic, where several species have been described from 

 the Upper Carboniferous. The situation is thus closely 

 comparable with Selaginella, whose herbaceous ancestors 

 were living alongside the related arborescent Lepidoden- 

 drales in Carboniferous times. 



