02 SURVEY OF THE REPRODUCTIVE PROCESS 



nating in a stigmatic point denuded of cuticle. All direct 

 contact of the ovule and pollen grain is thus prevented, and 

 impregnation is effected by the grains adhering to the 

 stigma, and from that sending down their tubes, through 

 the lax tissue of the canal of the style, into the micropyles 

 of the ovules in the germen below. No process of cell- 

 formation has been observed in the interior of the pollen 

 grain, and it is probable that its first protrusion is due 

 simply to an endosmotic action, causing the contents en- 

 sheathed in the extensible inner wall to be protruded in 

 finger-like processes through perforations at points where 

 the outer cellulose coat has given way. But the farther 

 advance of the tube must be effected by a proper growth, 

 for its extension soon comes many times to exceed the size of 

 the grain from which it was emitted. The ovule consists 

 at first of a cellular nucleus, round which a double coat 

 grows up, leaving only at the apex the pore termed the 

 micropyle, and within which a cell cavity or embryo-sac is 

 afterwards formed. This sac acquires much greater size in 

 some cases than in others, and occasionally protrudes from 

 the micropyle in the form of an " ovule-tube.* It contains 

 only a semifluid granular matter, or at most a mass of very 

 delicate cells. Among the contents, however, are generally 

 seen two or three particles more conspicuous than the 

 others, which have received the name of " germinal vesicles," 

 but which some observers consider to be mere unwalled 

 masses of protoplasm. ( Unlike the germ cell of the Coni- 

 ferae, which lies at the bottom of the " Corpusculum," they 

 occupy the space near the apex of the embryo-sac. The 

 precise relations between the point of the pollen-tube and 

 the embryo-sac were for sometime the subjects of much dis- 

 cussion. Schleiden, Geleznoff, De Bary, Wydler, Schacht, 

 and Tulasne, maintaining either an introversion or perfora- 



* Dr. Dickie, Ann. Nat. Hist., N.S., I., 260. 

 f Griffith & Henfrey, Micr. Diet., p. 482. 



