DEVELOPMENT OF THE SEED.
A proembryo with a rather long suspensor is developed from the fertilized egg. ([Pl. II, fig. 2]). The endosperm, which quite early forms a peripheral layer around the entire embryo sac, develops most rapidly about the embryo, which soon becomes thoroughly embedded in it. ([Pl. III, figs. 1 and 2.]) After the embryo has used up the endosperm in the micropylar end and has enlarged so much as to occupy nearly all of the space in this region, the development of the endosperm becomes more active in the chalazal end, and when the embryo is mature there is very little endosperm left.
The seed coat begins to form about the time of fertilization, although it apparently does not depend upon it, for in ovules where fertilization is prevented the outer integument undergoes the early modifications in the development of the seed coat before the ovule breaks down. The development of the seed coat is apparent first at the micropylar and chalazal ends, where the outer cells of the outer integument become much elongated and their outer walls thicken very soon after fertilization. The modifications in the development of the seed coat extend around the ovule from these points, involving at first only the outer or epidermal layer of cells which form the malpighian layer. Later, the cells just beneath the malpighian layer form the osteosclerid layer. Accompanying or closely following the formation of the osteosclerid cells, the remaining cell layers of the outer integument become modified into the nutritive and aleurone layer, and the seed coat is fully formed. Meantime the inner integument is practically all used as food.
Development of the Ovules and Pollen in Sweet Clover.
Fig. 1.—Section through the nucellus of an ovule of Melilotus alba, showing two megaspore mother cells. × 360. Fig. 2.—Median section through an ovule of Melilotus alba, showing the two cells resulting from the first division of the megaspore mother cell, and the relative development of the different parts of the ovule. × 300. Fig. 3.—Section through the nucellus of an ovule of Melilotus alba, showing two embryo sacs, one being more advanced than the other. × 360. Fig. 4.—Protoplasm of the pollen mother cell of Melilotus alba contracted and ready to undergo division. × 560. Fig. 5.—Pollen grains of Melilotus alba just formed, showing their dense cytoplasm and the presence of the mother-cell wall. × 560. Fig. 6.—a, Mature pollen grain of Melilotus alba, showing the binucleate condition at the time of shedding; b, surface view. × 560.
Fig. 1.—Median Section through an Ovule of Melilotus alba.