i-nj WELWITSCHIA 467 



reduced; the gynoecium consists of only one functional ovule in- 

 stead of an indefinite number as in Cycadeoidea. Church regards the 

 resemblance between the flowers of these two genera as an instance 

 of parallel development, which does not imply relationship. He 

 thinks there is ' no indication whatever of any relation to the car- 

 pellary flowers of the Angiosperms.' 



Hooker's account of the ovule has recently been considerably 

 extended by the important researches of Pearson^. The megaspore 

 ■consists of two regions, an upper fertile and a lower sterile portion ; 

 each is composed of ' cells ' with more than one nucleus ; some of the 

 ' cells ' of the fertile region grow upwards as tubes into the nucellar 

 cone where pollen-tubes are encountered and fertilisation ensues. 

 The nuclei in each ' cell ' of the sterile region fuse and uninucleate 

 cells are produced; this tissue now grows considerably in size and 

 cell-divisions occur resulting in the formation of an endosperm. 

 Pearson regards the free nuclei that are in the embryo-sac at the 

 time of septation into the multinucleate ' cells ' as all alike, and all 

 potential gametes. It follows, therefore, that the endosperm 

 formed in the lower portion of the sac is the product of fusion of 

 sexual nuclei; it is not a gametophyte or a sporophyte and Pearson 

 proposes for it the new term trophophyte, ' a bye-product resulting 

 from the fusion of potentially sexual nuclei and functioning in the 

 same manner as the prothallus of the lower seed-plants.' More 

 recent work by this author confiriiis his opinion that the endosperm 

 of Gneium is also a trophophyte. 



For an account of the anatomy of Welwitschia and Gnetum the 

 student is referred to original sources. Miss Sykes^ called atten- 

 tion to certain interesting characters, — the occurrence of reticulately 

 pitted protoxylem elements in the stem, the arrangement of 

 separate and not contiguous bordered pits in 1 — 2 rows on the 

 tracheids, and to the presence of concentric steles and inversely 

 orientated bands of vascular tissue in the stem and inflorescences 

 closely resembling Medullosean features. 



In certain respects the Gnetales are closer than the Conifers or 



1 Pearson (06^); (09). 



" Sykes (10); (lO^); Worsdell (01=); Pearson (12); Bower (81); (82); Hill and de 

 Frame (10); Boodle and Worsdell (94); Mary R. H. Thomson (16); Henriette 

 C. C. LaRiTifere(16). 



30—2 



