306 MR. W. GRIFFITH ON THE GENUS GNETUM. 



becoming necessarily much enlarged, and adhering firmly to the walls of the cavity it 

 lines. 



Although I have repeatedly examined the ovula of Gnetum with a view to the nature 

 of their impregnation*, I am not in possession of a single fact relative to its performance. 

 I must, however, mention, that the ovula of Gnetuni scandens and lepidotum do not 

 appear to be ever submitted to the action of the poUen derived from their proper male 

 flowers. In G. Brimoniamim this is obviated by the extreme smallness of the annulate 

 involucrum. My residence at the Botanical Garden of Calcutta, and the great and un- 

 varying kindness of Dr. Wallich, have put me in possession of some, I think, interesting 

 facts relative to the impregnation of Cycas, which it may not be amiss to state. I may 

 here observe, that the consideration of the mode of application of the poUen-tubes to the 

 apex of the nucleus in some plants possessing the ordinary structiu-e of ovarium, had led 

 me to suppose that these productions are merely organs of commujiication, developed on 

 account of the distance that necessarily exists in these plants between the stigmatic 

 surface and ovulum, and that hence, in Cycas and plants of a similar simple structure, 

 in which actual application of the pollen itself to the apex of the nucleus can obviously 

 take place, no pollen-tubes would be produced. 



In forming this view, I had not lost sight of the apparent penetration of the poUen- 

 tubes into the ovula of Asclepiadece, first observed by Dr. Brown ; but I supposed that it 

 might be the result of the anomalous formation of the nucleus ia these cases. Repeated 

 observation has since, however, taught me, that not only does application of the pollen- 

 granules to the apex of the nucleus of Cycas take place, but that pollen-tubes are like- 

 wise generated, although, as might be expected, much shorter than usual. Indeed, the 

 tubular membranous portion of the apex of the nucleus becomes actually crammed with 

 poUen-granules, from the lower and outer of which pollen-tubes are pretty generally 

 produced. 



The orifice of the envelope of Cycas has a callous and shining appearance; and 

 although I have often examined pollen-grains which had been in apposition with it for 

 some time, I have never seen any production of boyaux, except in the cavity of the apex 

 of the nucleus. 



Although I have by no means proved the necessity of the production of the pollen- 

 tubes in Cycas to ensure fecundation, I consider the fact of their production a strong 

 argument in favour of the idea that actual penetration does occvu- in every case in which 

 the application of the tubes to the apex of the nucleus can be conceived. The fact of the 

 production of the tubes Ukewise seems to me to put the nature of the bodies from which 

 they originate out of all doubt, and to prove the truth of Dr. Brown's remark (Memoir 

 cited, p. 30), that it would be quite gratuitous, on the grounds stated, to consider the par- 

 ticles contained in the thecae to be analogous to the fovUla. 



Dr. Brown, in his account of microscopical observations on the particles contained in 

 the pollen of plants, published in the ' Edinburgh Journal of Science,' vol. ix. p. 343, 



* The opening of the outer coat never presents a shining appearance. 



