CERTAIN STEUCTUliES IN THE GENUS GNETUM. 319 



Karsten records tliat in tlie species investigated by him *, fertilization occurs while 

 the sac is still nnseptate and all the nuclei are free. After fertilization the lower part 

 of the sac (in G. BumjpUanum and G. omlifoUum) becomes cellular by septation, the 

 *'ceU" freq^ueatly containing two or more nuclei. A similar condition is found also 

 in G. edule'\. At length, apparently, all the cells of the endosperm become 



unimicleate t. 



Lotsy found § that in G. Gnemon the lower (sterile) part of the sac becomes septate 

 before fertilization, and that at first some at least of the compartments into which 

 is divided are mnltinucleate. All eventually become multinucleate 



In G. Via f fertilization has not been described, and Lotsy was at first of opinion 

 that this species is parthenogenetic. More recently, however, he is inclined to interpret 

 differently the structures originally supposed to be apogametic embryos, and to regard 

 them as the homologues of the embryo-sac tubes of WehoitscUa**. However this 

 may be, in this, as in the other species described formerly by Lotsy and by Karsten, 

 the lower part of the sac becomes septate to form a solid endosperm tissue. 



In 1908, Coulter ft reinvestigated the embryo-sac of Gnetum Gnemon, He encountered 

 the difficulty which has so far confronted every one who has attempted to study 

 G7ietmi, viz. the absence of "many intervening stages" from his material. Coulter's 

 conclusions differ in some important respects from those of Lotsy. He describes 

 a clearly differentiated region of nucellar tissue at the base of the sac under the name 

 of « pavement tissue." At the time of the appearance of this tissue the embryo-sac 

 contains free nuclei throughout its length, and Coulter concludes that Lotsy's « antipodal 

 tissue is clearly this glandular pavement tissue developed in the chalaza " : m other 

 words, Lotsy's primary endosperm is a part of the nucellus which, as Coulter correctly 

 states, is later destroyed by the advance of the true endosperm W- 



Lotsy reaffirms his previous statements §§ without, however, being able to throw any 



new light upon the origin of the endosperm. 



In the related genus WelwltscMa \\ \\ the whole sac becomes septate, before pollination, 

 into multinucleate compartments. In those compartments which contain many nuc ei, 



,_, _,,^^ ^_^.. The uninucleate cells so formsd are primary endosperm cells, 



all of which arise in this way. These occupy almost the whole (frequently the whole) 

 of the chalazal two-thirds or three-auarters of the sac. In those compartments which 

 enclose fewer nuclei, the latter remain free and form the functional gametes ; the 

 of the compartments grow out into the nucellar cap forming the embryo-sac tubes 



The voungest ovules of G. africanum available are just at the stage at which he 

 upper half of the ovule emerges beyond the edge of the cupule (PL 31 fig. 1). In the 

 youngest of these the embryo-sac contained four free nuclei 11[. As the sac elongates 

 and tL nuclei increase in number, they depart from the regular peripheral arrangement 



* Karsten, 1893 (2). t Karsten. 1892, fig. 9 A. t Karsten 1892. fig. 21. 



11 Lotsy. 1899, p. 98. 1 Lotsy, 1903. 



*• Poiilter 1908. tX L. c. ; Coulter and Chamberlain, 1010, p. 3S'<5. 



nuclear 



alls 



§ 



Lotsy, 1911, pp. 350-352. ft Coulter, 1908. 



§§ 



nil Pearson, 1909. If <^/' 



SECOND SERIES. — BOTANY, VOL. VIII. 



3 



