318 BE. H. H. W. PEAESON ON THE MOEPHOLOGY OF 



grains ; some contained two, others three. All appeared to be q^uite free, and there 

 was no clear indication of the presence of a generative " cell " such as Karsten records 

 for later stages of various species and Lotsj* for G.moluccanum. The adult pollen- 

 grains of various Indo-Malayan species f and of G. africanum % possess three nuclei. 

 In all cases described, one of these (presumably prothaUial in character) disappears 

 before the pollen-grain reaches the micropyle. In G. Gnemon it would appear that all 

 three may persist until pollination has occurred, or one of the three may disappear as in 

 other species. "Whether the third always disappears before the pollen-grain germinates 

 is not known. 



G. The Macrospoee and Endosperm. (G. africanum.) 



The structure of the ovule of G. africanum has been described in detail 

 Mrs. Thoday §, that of G. Gnemon and of other species by various authors ||. Much 

 has also been written upon the embryo-sac, embryo, and endosperm, especially of 

 G. Gnemon^; there are, nevertheless, still very important gaps in our knowledge 

 of these structures. The origin and morphology of the endosperm is a subject 



3 



arding which further information is very desirable. Two types of endosperm 



known to occur in tlie Gnetales. One of these, a prothallus bearing archeg 

 characteristic of Ephedra. The second, arising from cells uniformly constituted by the 

 fusion of numerous nuclei, occurs in WelwitscMa. When the latter was first described **, 

 it was suggested that the morphological differences between these two types of endo- 

 sperm were of a more or less fundamental character; that Ephedra retains the 

 intrasporic prothallus of the lower spermaphytes ; that the endosperm of Welioitschia 

 is morphologically distinct from anything hitherto described for the gymnosperms, 

 representing, perhaps, an early term in an evolutionary series which has ended in the 

 highly specialized endosperm of the Angiosperms— a structure whose morphological 

 value is doubtful. However they are to be explained, the facts themselves are 

 important. Apart, then, from any morphological interpretation which may be placed 

 upon it, it is clearly a matter of some interest to determine whether in respect to its 

 endosperm Gnetum is to be classed with Ephedra or with WelwitscMa. 



Since previous investigators have left no room for doubt that the female gametes of 

 Gnetum are free nuclei as in Welioitschia, and that in both cases the accessory structures 

 of the archegonium are absent ft, it would appear probable, a priori, that the endosperm 

 of Gnetum also resembled that of WelwitscMa, Although the details of endosperm 

 formation were not followed either by Karsten or by Lotsy, their accounts, read in the 

 light of what is now known for WelwitscMa, contain a little internal evidence in favour 

 of a homology with the latter %%. 



• 



+ 



lotsy, 1S99, p. 94. f Karsten, 1893, p. 359 ; Lotsy, 1899, p. 94. 



Pearson, 1912. 



§ 



!l Strasburger, 1872, 1879; Karsten, 1892, 1893; Lotsy, 1899; Berridge, 1911, 1912; &c 



f 



tt a. lignier et Tison, 1912, p. 171. t* Pearson, 1909, pp. 356-358. 



** Pearson, 1909. 



