440 Ldn7ia:an Society. 



are entirely distinct. Their production he considers a vital action 

 excited in the grain by the application of an external stimulus, which 

 is afforded by the secretion on the surface of the stigma ; and they 

 derive nutriment either from the particles contained in the grain, or 

 from the conducting surfaces with which they are in contact. 



The first visible effect of the action of the pollen on the stigma is 

 the enlargement of the ovarium, which, in those cases where it was 

 reversed by torsion, untwists and resumes its original position. 



After impregnation the ovulum enlarges, the nucleus disappears, 

 probably from its acquiring greater transparency, and becoming con- 

 fluent wilh the substance of the testa : soon after, a minute speck 

 about the middle of the testa becomes visible, which is the commence- 

 ment of the future embryo. At this period a thread may be traced 

 from its apex nearly to the open end of the testa, consisting of a 

 simple series of short cells, the lowermost one of which is probably the 

 original state of what, from enlargement and deposition of granular 

 matter, becomes the opake speck or rudiment of the embryo; the only 

 appreciable changes in which are its increase in size and eventual cel- 

 lular structure. In the ripe state it forms an ovate or spherical body, 

 consisting of an uniform cellular tissue covered by a thin membrane, 

 the base of which exhibits no indications of original attachment at 

 that point, while at the apex the remains of the lower shrivelled joints 

 of the thread are still often visible. The embryo, therefore, would be 

 without albumen ; the germinating point its apex, or that where the 

 cellular thread is found ; and the seed and funiculus are without ves- 

 sels. 



Asclepiadece . — The mode of impregnation in this family was sup- 

 posed by Jussieu, Richard, Bauer, Treviranus, and by Mr. Brown, to 

 be indirect ; that is, that there was no immediate contact of the pollen 

 with the stigma, but that the fecundating matter was conveyed from 

 the mass through the arm and gland to the female organ. 



At a very early period Gleichen had observed that the pollen masses 

 were originally distinct from the glands, — a fact which Mr. Brown had 

 afterwards stated in 1809, and which had etjually been observed and 

 delineated by Mr. Bauer. Gleichen also states, that before the masses 

 unite with the glands they are removed from the cells of the anthera, 

 and implanted into the wall of the tube which surrounds the ovaria, 

 and that in this situation a white viscid substance hangs to them, 

 which consists of tubes containing globules ; and these tubes and 

 their contents he considers as the early preparation for the formation 

 of pollen. He remarks, that the tops of the styles are not originally 

 connected with the pentagonal body, and therefore, that impregnation 

 does not usually take place until the true stigmata, or those extre- 

 mities of the styles on vvhich vesicles and threads are observable, have 

 penetrated through the substance of the pentagonal body, and are on 

 a level with its apex. At the same time, he is disposed to believe that 

 insects may occasionally assist in the function by carrying the fecun- 

 dating matter directly to the stigmata, even before they enter the 

 pentagonal body. 



Sprengel in 1 793 asserts that insects extract the pollen ma.sses 



from 



