REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



of the nucleus there exists an ovoidal embryo- 

 sac, which owes it origin to the coalescence 

 of a vertical and axial series of cells. At this 

 period it contains only granules of starch and 



Fig. 185. 



Section of nucleus of ovule of Pinus Austriaca, in 

 the centre of ichich is observed the young embryo- 

 sac, 150 diam. 



mucus, the nucleus which it at first contained 

 having disappeared. It corresponds, as will 

 he seen as we proceed, to the internal mem- 

 brane of the ripe macrospore of the Rhizocar- 

 peae and Lycopodiaceae. The pollen grain in 

 theConiferaegenerally itself reaches the summit 

 of the nucleus by means of the wide micropyle. 

 From each grain emanates a tube, which pene- 

 trates for a short distance into the tissue of 

 the nucleus; not, however, until it has re- 

 mained for some time upon its summit. In 

 the meantime numerous free nuclei have be- 

 come visible in the embryo-sac, which imme- 

 diately afterwards " presents itself filled with 

 a large number of radially elongated cells, 

 which are arranged in a concentric layer." 

 These continue to multiply by septa in all 

 three directions, until the beginning of winter, 

 at which period the wall of the embryo-sac 

 is so delicate as to be indistinguishable, 

 During the winter months these cells undergo 

 no further change, except that their walls are 

 thickened by internal gelatinous deposition. 

 In the beginning of March of the second year, 

 both the gelatinous material and the cell-wall 

 disappear, the primordial sacs lying free in 

 the cavity of the embryo-sac, each containing 

 a large globular nucleus. Shortly after, the 

 nucleus of each cell disappears, and is re- 

 placed by two or four smaller ones, round 

 each of which new spherical secondary cells 

 are formed. The parent cell is dissolved, 

 snd immediately after, the same process is re- 

 peated in the secondary cells. While this is 

 taking place, the embryo-sac has increased to 

 twenty times its former volume ; its membrane 

 has become resistant and vitreous, while 

 throughout the whole ovule, with the excep- 

 tion of its summit, an active cell growth has 

 taken place. Towards the middle of May 

 the permanent cellular body which after- 

 wards fills the whole embryo-sac, originates 

 by the application, in successive layers, of the 



247 



cells contained previously in its cavity, to its 

 membrane. By the continuation of this pro- 

 cess, the sac becomes a second time filled with 

 cellular tissue.* Two or three of the cells sub- 

 jacent to the micropyle end of the embryo* 



Fig. 186. 



Section of naked ovule of Pinus maritime^ as 06- 

 sei-ved in January of the second year, 150 diam. 



The spherical embryo-sac is filled with cells, the 

 walls of which are already thickened by gela- 

 tinous deposition. Two pollen grains occupy the 

 funnel-shaped space between the wide micropvle 

 and the summit of the nucleus. The cellular 

 tissue of the latter is penetrated by two tubes 

 emanating from the pollen grains. At the dotted 

 line the tissue of the ovule becomes continuous 

 with that of the spermophore. 



sac now become larger than the rest, and are 

 destined to contain the germs of the future 

 embryos. As their development proceeds, 

 these bodies, the so-called corpuscula, assume 

 an elongated, oval form, and the space in- 

 tervening between their summits and the 

 membrane of the embryo-sac is occupied by 

 four small cells on the same level, which ar'e 



Fig. 187. 



Four cells which surmount the corpusculum of Pinus 

 sylvestris, seen from above, 200 diam. 



* To this tissue is commonly applied the term 

 albuminous body." It corresponds in its mode of 

 rierin with the " endosperm " ( 106.") of other 



origin with the 

 Phanerogamia 



endosperm" ( 106.) 



R 4 



