CEETAIN STEUCTUEES IN THE GEIS'US GNETUM. 317 



present (PL 31. fig. 6). "When this is the case the two supernumerary anthers decussate 

 with the others. The vascular structure of the axis bearing more than t\YO anthers has 

 not been investigated. 



The single envelope at the stage shown in fig. 7 has the form of an approximately 

 equal collar arising from the base of the axis. Later on, growth is confined to two 

 points at right angles to the plane in which the anthers lie, the posterior lobe so 

 produced overlapping the lower or anterior *. The origin of the sporogenous tissue and 

 tapetal layers is as described both for this species f and for G. africamim \ . The 

 elongation of the floral axis commences at about the same time as the periclinal divisions 

 arise in the primary tapetum layer. Up to this time the anther primordia arc sessile 

 in the minute axis, which bears the perianth-collar (fig. 7). The axis thus formed by 

 intercalary growth finally attains such a length that it may project as much as 3 mm. 

 beyond the edge of the subtending bract-capsule. Its elongation is probably not 

 completed wlien the dehiscence of the anthers commences. 



There has been some discussion as to the morphological character of this apparent 

 axis supporting the anthers. Strasburger regarded it and its homologue in the male 

 flower of Ephedra as being of the nature oE a true axis §. More recently, however, 

 evidence has been adduced in support of the view that the antherophore is not an axis 

 but a foliar structure formed by the fusion of what must be regarded as the filaments 

 of the anthers 11. This opinion is to some extent supported for Gnetim by the late 

 origin of this structure by intercalary growth. As pointed out by the authors cited, 

 this view has the advantage of offering a rational basis for the comparison of the male 

 flower of Wehvitschia with those of Ephedra and Gnetum. If the vascular knots which 

 Miss Berrido-e «![ describes as existing between the origins of the outer and middle 

 envelopes of the female flower are the vestiges of a whorl of microsporophylls, the male 

 flower of Gnetnm may well be derived from a hermaphrodite structure not essentially 

 diff-erent from that which persists in a reduced form in Wehcitscha. 



The development of the sporogenous tissue has not been traced in detail m this 

 species. Several stages, however, have been seen which correspond exactly with those 

 described for G. a/Hcanum **. As in the latter species, the microspore mother-ccU is 

 chambered (fig. 8), the limiting wall and the septa being in both cases very thick and 

 mucilaginous tt. Coulter determined the reduced number^of chromosomes m - 

 homotypic division of the mother-cell of this species to be 12 JJ. 



The germination of the pollen-grain has not been followed m detai ; its results 

 apparently the same as in ^. africanum ^^. The exine of the adult pollen-grai. 

 thick, covered externally by minute protuberances and without any obvious pores 

 their areas 1| «. The two pollen-grains shown in fig. D were found with others in 

 micropylar tube of the ovule of fig. 2. The nuclei were easily visible m most of th 



^ -rr t 1QQ1 f fi fio- 14 t Pearson, 1912, figs. 5, 6, 



Pea.on, 1912. pi. 60. fig. 2. ^^ !"■ f,t il 19 f Berridge, 1012. 



tl 



# 





Pearson, 1912. t^ ^'f- 



20. lilJ Of. 



II Thoday and Berrldge, 1912. "8 i^erridge, xjxj 



... T. „„... 1Q19 fi^. 15. tt Coulter, 1908. 



