Harold Henry Welch Pearson. 



Ixiii 



concludes that the fertile end of the prothallus is more specialised than the 

 corresponding region in Gnetum and that Welwitschia can no longer be 

 placed in the " enormous gap " which separates Gnetum gnemon from 

 Ephedra. 



A second expedition to Damaraland in 1906-7 enabled Pearson to fill up 

 gaps in the first account, and to confirm or correct his earlier impressions. 

 In the " Further Observations " he discusses the morphology of the 

 inflorescence and flower, particularly with reference to possible affinities 

 to the Bennettitales and the flowering plants. Additional facts are given 

 about pollination. The most interesting questions discussed concern the 

 nature of the tissue in the embi'yo-sac. The tubes which grow upwards 

 from certain cells in the embryo-sac to meet the descending pollen-tubes 

 are now termed embryo-sac-tubes in preference to prothallus-tubes, because 

 the examination of a more complete series of developmental stages led him 

 to regard the tissue in the macrospore as neither part of the gametophyte 

 nor of the sporophyte, but a distinct organism, which he names the 

 trophophyte. All the free nuclei in the embryo-sac, when septation begins, 

 are believed to be potential gametes. The sterility of the tissue in the 

 lower part of the sac is attributed to the fusion of the nuclei in the multi- 

 nucleate compartments ; this tissue, with nuclei possessing more than the 

 premeiotic number of chromosomes, is a by-product, resulting from the 

 fusion of potentially sexual nuclei, and it agrees with the prothallus of 

 the lower seed plants in providing for the nutrition of the embryo. In 

 Ephedra the endosperm is a true prothallus as in Conifers, but the 

 recognition of the true nature of the Welwitschia prothallus raised the 

 question of the morphology of the tissue in the embryo-sac of Gnetum and 

 the Angiosperms. It was primarily for the purpose of collecting material of 

 Gnetum africanum that Pearson organised and, with the assistance of the 

 Percy Sladen Trustees, successfully carried out his important expedition to 

 Angola and other regions in 1908-9. He found in the embryo-sac of 

 Gnetum a differentiation similar to that in Welwitschia ; after septation, the 

 compartments in the lower part of the sac contain five or more nuclei, but, 

 in the micropylar portion of the embryo-sac, septation does not occur. 

 Pearson also describes the germination of the microspore of Welwitschia 

 and the development and structure of the pro-embryo. His investigations 

 clearly demonstrate that Welwitschia and Gnetum are related to one another 

 more closely than either is related to Ephedra, a result which " might be 

 expected from the geographical distribution of the three genera." 



By his own researches, Pearson not only very considerably extended our 

 knowledge of Welwitschia and Gnetum, but his thorough and critical 

 investigations have thrown much new light on questions of general interest, 

 the nature of the tissue in the embryo-sacs of Gnetum and Welwitschia and 

 of the Angiosperms, the morphology of the flowers and inflorescences, the 

 embryogeny, and other subjects of morphological and biological interest. 

 Pearson's generous distribution of material to other botanists enabled them 



