OVULE, 



[ 5(37 ] 



OVULE. 



upon a style, and the pollen, falling- upon 

 the stigma, produces there its pollen-tubes, 

 which pass down through what is called 

 the couducting tissue of the style and upper 

 part of the ovary, on to the placentae, 

 whence they make their way to the micro- 

 pyles of the ovules. Plants exhibiting 

 these conditions are distinguished as Angi- 

 osPERMs or covered-seeded. 



The next phenomena which characterize 

 the development of the ovules of the An- 

 giosperms may be briefly given as follows. 

 The formation of the embryo-sac has already 

 been described. Shortly before the opening 

 of the flower, in most cases, this sac is more 

 or less denselj' filled with granular proto- 

 plasm, in which a variable number of nuclei 

 may be seen (PI. 47. figs. 1-7). About the 

 time when the pollen-grains are discharged 

 fi'oni the anthers, a number of minute, free, 

 globidar protoplasmic bodies may be dis- 

 covered in the embryo-sac, usually three 

 (^more rarely one) of these being crowded 

 into the upper end of the embryo-sac and 

 constituting what are called the germinal 

 bodies or masses (PI. 47. fig. 4), Others, 

 which often occur in the embryo-sac, are 

 generally collected near the bottom of it; 

 they are apparently characteristic of par- 

 ticular famih.es only; in some plants they 

 are very large, as in the Crocus. About 

 this time the embryo-sac often exhibits 

 asymmetrical growth, forming pouches or 

 processes, sometimes at the summit, some- 

 times at the base. 



"When the pollen-grains fall upon the 

 stigma, they protrude their pollen-tubes 

 (see PoLLEx), which pass down through 

 the conducting tissue, and enter the micro- 

 pyles of the ovides. Wben they reach the 

 apex of the embryo-sac, they either stop, 

 often swelling a little, or they pass down a 

 short way over its side (PL 47. tig. 5) ; very 

 rarely two pollen-tubes are found engaged 

 in the micropyle of the same ovule. It is 

 not absolutely known whether the cavities 

 of the pollen-tube and the embryo-sac 

 become actually continuous by absoi-ption 

 of the walls at the point of attachment ; it 

 is generally believed not, but we feel some i 

 doubt on this point. The essential point of 

 the process is the intermixture of the con- ' 

 tents of the pollen-tube ^\4th the substance j 

 of the germinal body. In the higher [ 

 Cryptogamia and in the Algae, the impreg- | 

 nation is of a similar nature ; but there the 

 germ-masses are fertilized by the agency of ; 

 spermatozoids, which make their way to 1 



them, cither constituting or carrying the 

 impregnating matter, Avhich in the case of 

 the pollen-tube is a liquid, containing fine 

 granules, but exhibiting no trace of active 

 spermatic bodies, except that refractive gra- 

 nules are sometimes seen in active motion 

 in the end of the pollen-tube. 



Soon after the pollen-tube has reached 

 the point of the embryo-sac, one (rarely 

 two, giving rise to Polyembryony) of the 

 germinal bodies becomes invested by a cel- 

 lulose membrane {(/erm-cell), and usually 

 changes from a spherical to an oval form, a 

 transverse septum soon dividing it into two. 

 Most frequently the elongation continues, 

 with a successive formation of septa, until 

 the nascent embryo appears as a rounded or 

 oval cellule suspended at the base of a 

 simple confervoid filament (suspensor) ; in 

 other cases the formation of the first trans- 

 verse septum is followed by the expansion 

 into two globular cellules connected by a 

 naiTow neck, the upper, almost devoid of 

 contents, constituting the suspensor (Pota- 

 moffefon, Zannichellia) ; in Orchis, the upper 

 of the first two cells grows upwards and 

 outwards, as a blind septate confervoid fila- 

 ment, through and beyond the micropyle of 

 the ovule. In Tropceolum and Zea the sus- 

 pensor becomes more complex, by formation 

 of perpendicular septa. In all cases the 

 end or embryonal cell, at the point of the 

 suspensor, which always appears densely 

 filled with protoplasm, ultimately enlarges, 

 and by segmentation is converted into the 

 embryo (PI. 47. fig. 6). 



During the early development of the em- 

 bryo, the embryo-sac is often found more 

 or less densely filled with free cells formed 

 from its protoplasm (ewf?os^?^;-m-cells) . These 

 are fi-equently absorbed, and disappear du- 

 ring the growth of the embryo, this ulti- 

 mately filling the embryo-sac ; while in 

 other cases they persist and multiply, form- 

 ing the ALBUMEN of the seed. In the 

 Nymphaeaceae these cells remain, forming 

 an inner Endosperm or Albumen, in addi- 

 tion to that formed from the body of the 

 nucleus. In other cases (those of exalbu- 

 minous seeds) the embryo not only displaces 

 these internal endosperm-cells, but in the 

 course of its growth causes the absorption 

 of the tissue of the nucleus, and idtimately 

 constitutes the entire seed, enclosed only 

 by the true integuments. The remaining 

 characters are given under Albumen and 

 Embryo, 

 Tulasne is in doubt whether the germinal 



