THE GYMNOSPERMAE : CYCADALES, ETC. 



777 



TUBE OF INNER 

 INTEGUMENT 



WING OF OUTER 

 NTEGUMENT 



NUCELLUS 



EMBRYO SAC 



Fig. 779. — Wehvitschia mirahilis. Longitudinal section of the ovule in the 

 plane of the integumental wings, at right angles to Fig. 778. 



(From Church, " Phil. Trans. Roy. Soc," B, 205, by permission 0/ the Royal Society.) 



divisions are simultaneous and are repeated until approximately 1,024 nuclei 

 are formed {i.e., ten divisions). There is no central vacuole, and all the 

 nuclei are evenly distributed throughout the sac. Irregular wall formation 

 then begins, enclosing groups of several nuclei in each cell, but by successive 

 nuclear fusions, which occur sooner in the apical portion, these are finally 

 reduced to one in each cell. 



There is no trace of archegonia. Instead, certain cells of the apical part 

 of the prothallus break through the megaspore wall and grow up as tubes 

 into the nucellus, towards the pollen chamber formed at its apex, where they 

 meet and fuse with the pollen tubes descending from germinating pollen 

 grains (Fig. 780). A nucleus from a prothallial tube passes into the pollen 

 tube and there fuses with the generative nucleus. 



The cytology of all this is highly peculiar. If the nucleus in the pro- 

 thallial tube has been formed by the fusion of several nuclei it must be 

 polyploid and some reduction of chromosomes must follow or accompany 

 fertilization. In the vegetative cells of the lower part of the prothallus, mitoses 

 occur following the nuclear fusions, and they show at first a very large number 

 of chromosomes, which, however, seems to get less after successive divisions. 

 The details are unknown. The question is, can we still apply the term 

 gametophyte to a structure whose cells are no longer monoploid [x) but 

 polyploid {nx) in nature ? 



