CEETAm STEUCTUEES IN THE GENUS GNETUM. 325 



gametes and produced embryo-sac tuLes became uninucleate endosperm cells 

 by nuclear fusion. 



7. Under certain circumstances, no endosperm is formed at all, but, with the 



exception of a few which succumb to the greater activity of their neighbours, 

 all the nuclei remain free and every compartment produces an embryo-sac tube 



of the normal type. 



8. In two cases (PI. 32. figs. 16, 17) about six gametes in an advanced embryo-sac 



tube have fused to form a nucleus in all respects like that of a primary 



endosperm cell. 



9. A small tissue found in an embryo-sac tube and consisting of seven cells is 



interpreted as an endosperm formed by such a nucleus as those referred to 



in §8. 



It is submitted that, when these grounds are substituted for those given by Dr. Lotsy, 



it is difficult to avoid the conclusion that the unf used nuclei of the eventually sterile 



part of the sac are not only homologous with, hut also potentially the physiological 



equivalents of, the functional gametes. They are potential gametes; they cease to be 



these when they fuse. 



The same conclusion for the fusing nuclei of Gnehm is founded partly on fact, partly 

 on analogy. But until complete proof is forthcoming, it may reasonably be adopted. 

 It has already been adopted by Coulter and Chamberlain, who write :— " The free nuclei 

 (of the Gnetum sac) are potential egg-nuclei, although a group near the antipodal end of 

 the sac may be as distinctly vegetative as are the antipodal cells of Angiosperms " *. 



When this nuclear fusion in Welwitschia was first described, reference was made to 

 the occurrence of multinucleate cells in the prothallia of other gymnosperms (includin 

 Uphedra) and to the nuclear fusion which occurs in these cells in Taxus f. As Lotsy 

 appears to regard this fusion as in all respects similar to that which occurs in Wel- 

 witschia— imdi now in Gnetum^-ixkviYiQV reference may be made to it. 



Multinucleate cells occur in the prothallus in many Conifers §. These may be due 

 either to the formation of multinucleate alveoli or to the occurrence of free nuclear 

 division in uninucleate endosperm cells. In most cases this condition, however attained, 

 is not followed by nuclear fusion. If a multinucleate ceU becomes uninucleate, it does 

 so, not hy the fusion of its nuclei, but by the formation of new dividing walls; otherwise 

 it remains multinucleate. Saxton, discussing these cases, concludes that *' there is no 

 evidence that the (multinucleate) character is of any importance, except as a nutritive 



adaptation." 



There are two conspicuous cases in which the multinucleate condition in the prothallial 



cells of the lower Gymnosperms is followed by fusion, and they are worthy of attention. 



In Ephedra, Strasburger |1 states, free nuclear division occurs in the neighbourhood 



of the archegonia, not only in the endosperm hut also in the nucellus. The division is 



• Coulter and Chaml^erlain, 1910, p. 388. t Pearson, 1909, p. 350. J Lotsy, 1911, p. 3iO. 



§ Lawson, 1904, 1907, 1909; Saxton, 1909, 1910, 1913; Sinnott, 1913. 

 II Strasburger, 1880, pp. 106, 107. 



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