EMBRYOGENESIS IN LI: PTOS POR ANGI ATE FERNS 151 



Trichonumcs consists of a llattcncd thallus like liiat of many Icpto- 

 sporangiate ferns; but in other species it may be filamentous and 

 branched like a moss protoncma. In Hynicnophvl/uni the prothallus is 

 typically a branched, strap-like structure, usually one cell in thickness 

 except for the cushions in which the archegonia are formed. The 

 embryogeny in those species in which the prothallus is either filamen- 

 tous, or in the form of a thin narrow ribbon, seems likely to be of 

 special interest; for in them the effect of the gametophyte on the 

 developing zygote may be expected to be somewhat different from that 

 exercised by a more massive prothallus. Unfortunately, little is known 

 about the embryogeny in the Hymenophyllaceae. However, Holloway 

 (1930, 1944) has given an account of the prothallus and embryonic 

 development in one species, Cardiomanes reniforme {Trichomanes 

 reuifonue). In this species the slowly growing gametophyte consists 

 of narrow branching ribbons, often one cell in thickness. Completely 

 filamentous forms were also observed. The form of the gametophyte 

 is thus very variable, especially in its earlier stages of development. The 

 archegonia are typically found in groups on the upper and lower sides 

 of well-formed ribbons, Fig. 35 bis a. In all, Holloway has reported 

 on the structural development of some eighty embryos, ranging from 

 the fertilised ovum to the beginning of growth in the young sporophyte. 

 All his illustrations, however, show the neck of the archegonium 

 directed upward, though the configuration of some of the embryos 

 portrayed suggests that they had been formed in downwardly directed 

 archegonia. The main points which have emerged from this valuable 

 study may be summarised as follows. Fig. 35 bis. (1) The first partition 

 wall in the segmentation of the zygote may be at right-angles to the 

 long axis of the archegonium, or obliquely inclined to it ; but it is never 

 quite in the plane of this axis, as in leptosporangiate ferns. In two 

 associated archegonia, on opposite sides of the same cushion, one had 

 the first wall obliquely transverse, while the other had it so strongly 

 inclined as to be almost vertical. (2) The epibasal (or neck) segment 

 and the hypobasal segment then divide by walls at right-angles to the 

 first or basal v/all; but some anomalous embryos consisting of a 

 linear group of three cells were also observed. (3) After the quadrant 

 stage there is no regularity in the ensuing divisions, and the apical cells 

 of the primary organs cannot be distinguished until considerable 

 further development has taken place, i.e. the embryo consists of a 

 histologically undifferentiated globular or ellipsoidal tissue mass. 

 (4) When at length the apical, or initial, cells of the first leaf (cotyledon) 

 and first root can be distinguished, they are seen to be quite inconstant 

 in their relation either to the neck of the archegonium or to the longi- 

 tudinal direction of the gametophyte ribbon. A little later, the initial 



