CEETAIN STEUCTUEES IK THE GENUS GKETUM. 329 



he writes : — :" Comme Pearson n'argumente pas cette assertion, nous ne pouvons com- 

 prendre pourquoi il considere les noyaux du sac du WelwitscUa entrant en fusion, 

 comme noyaux potentiels, ou noyaux en repos, et pourquoi 11 ne considere pas comme 

 tels les noyaux des Angiospermes." In the case of Welwitschia there is some evidence 

 that the nuclei which fuse are potential gametes, for if by chance fusion does not occur 

 in a compartment embedded in the endosperm, its walls may grow out into the nucellus 

 as an embryo-sac tube *. In EuphorUa virgata f, in which sixteen free nuclei appear 

 in the sac (megaspore), it is not known that more than one actually retains the power of 

 functioning as a gamete. If the remaining fifteen have lost that power, then they may 

 be described as " the arrested representatives of nuclei which in earlier forms were 

 potentially sexual." Therefore, EupJiorhia virgata being substituted for Peperomia, the 

 opinion expressed in the sentence above quoted is almost identical with that advocated 

 by Dr. Samuels. 



Coulter I, regarding the endosperm of the Angiosperm as a gametophyte, considers 

 that " conditions in the embryo-sac favour fusions of any free nuclei, in any number and 

 of any origin." This conclusion will probably not be questioned. And that " there is 

 no necessary phytogeny of such a performance" may also be true for this case, but 

 it seems to demand more evidence in support of it than the author has been able to bring 

 together. Eefore it can be accepted as the whole explanation of the phenomena, it will 

 be necessary to show why a set of different conditions leads also to nuclear fusion in 

 Gnetum and in Welwitschia^ and why this fusion produces physiological results so closely 

 comparable to those achieved in the Angiosperm. It would appear that the intracellular 

 conditions in the multinucleate alveolus of Cryptomeria japonica §, for example, are as 

 favourable to nuclear fusion as those which exist in the Angiosperm sac and in the multi- 

 nucleate compartment of Gnetum. And yet fusion does not occur. And if the fusion 

 itself possesses no morphological significance, the phylogeny of the nuclei which fuse is 

 still to be determined. Among the very diverse views that have been entertained, the 

 possibility that the polar nuclei of tbe Angiosperm may be morphologically (as well as 

 functionally) the representatives of the fusing nuclei of Welwitschia and Gnetum, has 

 perhaps received too little attention. 



While the whole question is involved in much obscurity, it is surely not desirable at 

 the present stage of the enquiry that the search for a primitive type of endosperm, from 

 which that of the Angiosperm may be derived, should cease — which would tend to follow 

 from the assumption that the fusion of the polar nuclei, with or without a sperm nucleus, 

 means nothing more than that they could not do otherwise in the conditions prevailing 

 m the embryo-sac. Widely different as are the views adopted by the many authors who 

 have considered the question, nearly all agree that the phenomenon is one the expla- 

 nation of which is to be found in phylogeny ||. There is at least a prima facie case for 



I r. 



* Pearson, 1909, p. 3(T5, text-figure. 



T lYhen the comparison between Welwitschia and Peperornia was instituted, Brown's (1908) results were 

 unknown to me. Tbe fact that the Peperomia sac represents four spores robs the argument of its cogency. The 

 same applies to Pene<Ma (Stex^hens, 1909) and Gunnera (Samuels, 1912). EnjJiorUa virgata (DessiatoflF, 1911) i» 

 free from this objection. . " 



X Coulter, 1911. § Lawson, 1904, fig. 23. || Cf. Davis, 1904. 



\ 



3b2 



