OVULE. 



[ 481 ] 



OVULE. 



ming folded or bent upon itself, without 

 disturbance of the relative positions of the 

 hilum and chalaza, while micropyle is brought 

 down, as in the anatropous ovule, to the 

 vicinity of the hilum. This form is termed 

 campylitropous (fig. 551) . Other conditions 

 occur less frequently, among which is the 

 ampJiitropous form (figs. 555 & 556). 



During these developments the embryo- 

 sac also undergoes various changes. Some- 

 times, as in the Orchidaceae, it expands so 

 as to oblitei-ate all the tissue of the nucleus, 

 and appears like a simple sac enclosed by 

 the coats ; in the Scrophulariaceae and other 

 orders it produces peculiar lobes or pouches 

 at various points ; in the Santalaceae it grows 

 out from the summit of the nucleus, as a 

 free, naked, tubular process, &c. 



Up to this point the diff'erences in ovules 

 are such as may be termed secondary, but a 

 primary distinction now comes into view, 

 connected also with a difference in the 

 external conditions, affording grounds for 

 the division of the Flowering Plants into two 

 great classes. In the Coniferse and Cyca- 

 daceae the ovules are developed upon open 

 carpels, and consequently the micropyle may 

 receive the pollen-grains immediately, when 

 expelled from the anthers. Plants exhibiting 

 this condition are termed Gymnosperms, 

 or naked-seeded. In the Dicotyledons and 

 Monocotyledons the carpels are always closed 

 up into cases or ovaries, surmounted by a 

 stigma, sessile or elevated upon a style, and 

 the pollen, falling upon the stigma, produces 

 there its pollen-tubes, which pass down 

 through what is called the conducting tissue 

 of the style and upper part of the ovary, on 

 to the placentae, where they make their way 

 to the micropyles of the ovules. Plants ex- 

 hibiting these conditions are distinguished 

 as Angiosperms or covered seeded. 



The next phaenomena which characterize 

 the development of the ovules of the Angio- 

 sperms 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 densely filled with granular proto- 

 plasm, in which a variable number of nuclei 

 may be seen (PI. 38. figs. 1-7). About the 

 time when the pollen-grains are discharged 

 from the anthers, a number of minute free 

 cells may be discovered 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 vesicles (PI. 38. fig. 4). Others, 



which often occur in the embr}'o-sac, are 

 generally collected near the bottom of the 

 embryo-sac ; they are apparently character- 

 istic of particular families only ; in some 

 plants they are very large, as in the Crocus. 

 About this time the embryo-sac often exhi- 

 bits asymmetrical growth, forming pouches 

 or processes, sometimes at the summit, 

 sometimes at the base. 



When the pollen -grains fall upon the 

 stigma, they produce their pollen-tubes (See 

 Pollen), which pass down through the 

 conducting tissue, and enter the micropyles 

 of the ovules. When 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 (PI. 38. fig. 5). Not un- 

 frequently 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 absorption 

 of the walls at the point of attachment ; it 

 is generally believed not, but we have recently 

 had occasion to feel some doubt on this 

 point. Soon after the pollen-tube has 

 reached the point of the embryo-sac, one 

 (rarely two, giving rise to Polyembryony) 

 of the germinal vesicles becomes richer in 

 protoplasm, and usually changes from a sphe- 

 rical to an oval form, a transverse septum 

 soon dividing it into two. Most frequentty 

 the elongation continues, with a successive 

 formation of septa, until the nascent embryo 

 appears as a rounded or oval cellule sus- 

 pended at the base of a simple confervoid 

 filament {suspensor) ; in other cases the for- 

 mation of the first transverse septum is 

 followed by the expansion into two globular 

 cellules connected by a narrow neck, the 

 upper, almost devoid of contents, constitu- 

 ting the suspensor {Pofamogeton, Zanni- 

 chellia) ; in Orchis, the upper of the first 

 two cells grows upwards and outwards, as a 

 blind septate confervoid filament, through and 

 beyond the micropyle of the ovule. In Tro- 

 pcEolum and Zea, the suspensor becomes more 

 complex, by formation of perpendicular septa. 

 In all cases the end-cell {embryonal vesicle), 

 at the point of the suspensor, which always 

 appears densely filled with protoplasm, 

 ultimately enlarges, and by segmentation is 

 converted into the embryo (PI. 38. fig. 6). 



During the early development of the 

 embryo, the embryo-sac is often found more 

 or less densely filled with free cells formed 

 from its protoplasm (ewc?05perm-cells). These 

 are generallv absorbed and disappear during 



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