386 FOURTH GROUP. SEED-PLANTS. 



definite outline in the apparently homogeneous tissue, their position being indicated 

 only by their embryo-sacs. 



Before the formation of the integuments certain differentiations take place in 

 the nucellus introductory to the formation of the embryo-sac (macrospore), differen- 

 tiations similar to those observed in the macrosporangia of the Coniferae and in other 

 sporangia, and especially in the microsporangia or pollen-sacs of Angiosperms. The 

 researches of Strasburger and Warming have made us acquainted with these important 

 processes, and have supplemented and corrected Hofmeister's earlier observations. 

 A good example is afforded by Polygonum divaricatum which was carefully 

 examined by Strasburger. The ovule occupies the summit of the floral axis, and is 

 therefore terminal upon the axis. The hypodermal terminal cell of the axile row of 

 cells of the nucellus (Fig. 319/3) is the archesporium or mother-cell of the embryo-sac 

 or macrospore. This cell divides into an upper and a lower and larger cell. The former 

 (Strasburger's tapetal cell) may be termed the primary tapetal cell 1 (Fig. 319 /); it 

 increases considerably in size in many ovules and divides repeatedly, so that the 

 archesporium (Fig. 319 em] is sunk deep in the tissue of the nucellus. There it 

 divides, as is shown in Fig. 319 // and ///, by transverse (anticlinal) walls first 

 of all into two and then into four cells, while the primary tapetal cell is also divided by 

 a longitudinal and a transverse wall. The transverse walls in the mother-cell of the 

 embryo-sac are marked by their power of refracting light, and have the appearance of 

 being swollen. Of the four cells into which the mother-cell is now divided, the lower 

 only undergoes further development and becomes the embryo-sac or macrospore. The 

 protoplasm of the three upper cells (cap-cells} becomes grumous and strongly refractive, 

 while the lower cell as it grows squeezes them together, and at the same time has the 

 disorganising effect, commonly observed in sporangia, on the cells formed from the 

 primary tapetal cell, the lowest of which must be considered as the tapetum ; there 

 is left at last only a cap of a strongly refractive substance, covering the apex of the 

 embryo-sac (macrospore), and the embryo-sac as it enlarges exerts a similar destruc- 

 tive influence on the adjacent lateral cells of the tissue of the nucellus (Fig. 319 V), 

 resembling in this the macrospores of Isoetes, the Cycadeae and Coniferae 2 . Mean- 

 while changes are taking place in the embryo-sac or macrospore itself. The nucleus 

 divides and one of the two daughter-cells moves into the upper or micropylar end of 

 the embryo-sac, the other into the lower extremity. The upper nucleus gives rise to the 

 egg-apparatus, the lower to the antipodal cells ; each of them divides again (Fig. 319 

 VI) and the division is repeated in each of the daughter-cells. There are now 

 therefore four nuclei at the upper and four at the lower end of the embryo-sac. 

 Three out of each group of four now become invested with protoplasm and are naked 

 cells. The three upper cells 3 , which have no cell-wall, together form the egg- 

 apparatus, and one of the three lying deeper in the embryo-sac than the other two 

 (Fig. 319 VII, o) is the oo sphere, the others which assist only in the process of 

 fertilisation are termed the synergidae. The three lower cells, which in this and in 

 many other cases are invested with a wall of cellulose, are the antipodal cells ; they 

 take no part in the processes which follow, and they eventually disappear. In both 



1 See on page 363. - See pages 294 and 332. 



;: Formerly known as germinal vesicles. 



