328 DE. H. H. W. PEAESON ON THE MOEPIIOLOGT OF 



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frequently produced by tlie fusion of gametophyte nuclei of female origin only. The 

 monstrous form of the sporophyte might well he accounted for by the large number of 

 nuclei entering into fusion *. 



The third alternative, namely, that the endosperm of Welwitschia (and therefore of 

 Gnetum) is neither a gametophyte nor a sporophyte, has been suggested f. It was 

 regarded as a distinct morphological entity, appearing for the first time in the higher 

 Gymnosperms and persisting in a highly modified form in the Angiosperms, " a by- 

 product resulting from the fusion of potentially sexual nuclei" and functionally 

 replacing the prothallus of the lower spermophytes. 



It is admitted that the evidence for this last view is small, and may prove to be quite 

 inadequate. Except that it seems impossible to refer this endosperm to the gametophyte, 

 it is probably wiser at this stage not to give it a definite morphological label. But the 

 fact that it occurs in Gnetum, as well as in WelwitscUa, still further emphasizes the 

 necessity of recognizing that its morphological importance is not negligible. These are 

 the only genera among living Gymnosperms in which an endosperm constituted in this 

 manner is known to exist. The only other known case of the origin of an endosperm 

 from nuclear fusion is found in the Angiosperms. Since, also, there is a general consensus 



of opinion that Wchcitschia and Gnetum are nearer to the Angiosperms than are any 

 other living Gymnosperms, the possibility that the peculiar endosperm of the Angio- 

 sperms had its origin in a nutritive tissue not essentially different from that of these 

 two genera cannot be overlooked. In the light of present knowlege it is hardly possible 

 to discuss adequately the morphology of the endosperm of the higher group without any 

 reference to that of the GnetoidejB J. 



The relationship of the endosperm of Welnnfschia to that of the Angiosperm has 

 recently been discussed by Samuels §. The author accepts the view that the free nuclei 

 of the Angiosperm sac are phylogenetically related to the free nuclei of the Welwitschia 

 sac :— " La formation libre des noyaux dans les Gnetacees est assurement identique a 

 celle qui a lieu chez les Angiospermes " " II n'existe, a mon avis, aucune 

 essentielle entres les produits de fusion du Weltcitschia et ceux des Angiospermes 

 He considers that the prothallus has disappeared both from the upper Gymnosperms 

 and from the Angiosperms If, but he does not otherwise pronounce a definite opinion 

 upon the morphology of the endosperm. 



Samuels quotes the following expression used in comparing the sacs of Welwitschia 

 and :Peperomia^*:~^^T}\icvQ is no evidence that the fusing nuclei of Teperomia are 

 potential gametes, but they are probably to be regarded as the arrested representatives 

 of nuclei which in earlier forms were potentially sexual." Commenting upon this 



difieren 



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It may be noted here that pro-embryos of the ordinarj- type are stated to be formed in Ephedra distachja by 

 the fusion of nuclei derived from the jacket-ceUs (Berridge and Sanday, 1907). 



t Pearson, 1909. j Cf. Coulter, 1911. § Samuels, 1912. 



11 Samuels, 1932, p. 85. A misinterpretation of the use of the word " trophophyte " may here be corrected. 

 The author says (p. 85) that the endosperm of Welwitschia, " pour le distinguer des noyaux de I'albumen des Angio- 

 spermes, a ete appele par Pearson trophophyte." On the contrary, the term trophophyte was applied to the fusion- 

 product in both Welwitschia and the Angiosperm, to distinguish it from the prothallus of the lower Gymnosperm. 



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** Pearson, 1909, pp. 378, 379 ; Samuels, 1912, p. 97. 



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