CEETAIN STEUCTUEES IN THE GENUS GKETUM. 



327 



more compact cells, and a more extensive peripheral region of larger looser cells " *. 

 In its physiological results it is to be compared with the fusion of the polar nuclei of the 

 An^iosperms, not with the fusions which characterize the degenerating of the endosperm 

 of Taccus and of the endosperm and nucellus of Ejihedra. The former is a fusion which 

 precedes organization ; the latter, disorganization. 



Those who favour the latter comparison, must at least accept the onus of proving that 



real 



Lotsy has passed over the difficulties without any reference to tliem 



whether 



Lotsy f would regard each multinucleate compartment of the Welicitschia sac- 

 its nuclei are destined to function as gametes or, by fusion, to constitute a primary endo 

 sperm cell — as homologous with a Coniferous multinucleate alveolus of the " Sokolowa 



type 



If the free nuclei of the latter are potential gametes, this may w el*l be 



But the 



difficulty is that, so far, they are not known to fuse ; those of WehcitscUa and Guetum 



do 



And if the nuclei of the '^ Sokolowa " alveolus did fuse and constitute a cell 



It 



divided to form a tissue, would this tissue be gametophytic ? In Taxus and Ephedra 

 the fused nuclei lose one of the most distinctive characters of the generation in whose 

 disorganization they ap^^ear, and they leave no descendants. 



If, then, the endosperm of WelicitscJiia and Gnetum is not merely an adaptation of 

 fusion of degeneration occurring in a few of the lower Gymnosperms, ^vhat is it ? 

 must be a modification of the gametophyte, or of the sporophyte, or something different 

 from both. In discussing this question it is well to bear in mind that no criterion dis- 

 tinguishing the gametophyte from the sporophyte, applicable to all cases, is yet known. 

 But when the life-history includes fertilization, there must be a compensating reduction 

 of chromosomes. Consequently, in such cases the number of the chromosomes is £ 



character of impor 



endosperm nuclei in these two genera contain 



larg 



number of chromosomes than even those of the sporophyte 



Another character which 



IS 



ay 



o 



ded as decisive when fertilizat 



bears the gametes, 

 gametes. Much has 



The endosperm in these g 



occurs, is that 

 does not in any 



the gametophy 



bear the 



about the differences between the prefertilizati 



endosperm of G. Guemon (such as also occurs in G. qfricamim and Welwitschia) and the 

 endosperm formed after fertilization in other species of Gnetum described by Karsten. 

 But, so far as is known, all species of Gnetum resemble Welicitscliia in this, namely, that 

 the endosperm is not formed until after the constitution of the functional gametes. That 



being 



so, no great stress can be laid upon the occurrence of fertilization before or aft 



the formation of the primary endosperm 



In the ca^es investigated the endosperm docs 



bear the gametes, but is itself produced by them J 

 ded as a gametophyt 



For these reasons it is not 



Some writers on the morphology of the endosperm of the Angiosperms have argued 

 that this tissue, arising normally from the fusion of two or three (sometimes more) nuclei 

 of the a7-gen oration, is a monstrous sporophyte §. Some of the reasoning applied to 



these cases applies also to 

 participates in the fusion of these 



• Coulter, 1908, p. 45. 



G. Brown and Sharp, 1911, p. 449. 



D SERIES. — BOTANY, VOL. VIII. 



Gnetum and Welwitsch 



It 



male 



but it is well known that a sporophyte is 



t Lotsy, 1911. 



§ Le iloTinier, 1887; Sargant, 1900; East, 1911. 



3b 



