174 THE MORPHOLOGY OF PTERIDOPHYTES 



Within the microsporangium, the sixty-four microspores 

 come to He at the periphery of a single frothy massula (Fig. 

 27K). They remain within the sporangium throughout and, 

 as they germinate, the male prothalli project all round. Each 

 male prethallus contains two antheridia (Fig. 27L) pro- 

 ducing a total of eight antherozoids. 



The megaspore, too, remains throughout within the 

 sporangium, after it has become detached. The female pro- 

 thallus protrudes, as a cap of tissue from which extend 

 backwards two narrow horizontal wings, or *stabiHzers\ 

 Several archegonia develop, in a row, across the upper side 

 of the projecting cap, each with a short neck, a neck canal 

 cell with two nuclei, and a ventral canal nucleus (Fig. 27M). 



Fig 27N illustrates a young sporeHng, still attached to the 

 female prothallus within the megasporium, and shows the 

 peculiar development of a 'column' (i), which separates the 

 first leaf (2) and the stem (3) from the foot, which remains 

 embedded in the prothallus. The early stages of segmenta- 

 tion of the zygote are pecuHar and their morphological 

 relationships are not fully estabUshed. At no stage is a root 

 primordium distinguishable. 



If the relationships of the Marsileaceae are obscure, those 

 of the Salviniales are even more so. The gradate origin of the 

 sporangia within the sporocarp, the intercalary growth of 

 the receptacle in Azolla and the vestigial oblique annulus 

 have led to the suggestion that the group has affinities with 

 the Hymenophyllaceae. However, this hardly seems accept- 

 able, in view of the many extraordinary features that mark 

 them off from all other ferns. 



