653. Macrospore and embryo sac.—In trillium the three carpels are united into one, and in dentaria the two carpels are also united into one compound pistil. Simple pistils are found in many plants, for example in the ranunculaceæ, the buttercups, columbine, etc. These simple pistils bear a greater resemblance to a leaf, the margins of which are folded around so that they meet and enclose the ovules or sporangia.
Fig. 389.
Young ovule (macrosporangium) of podophyllum. n, nucellus containing the one-celled stage of the macrospore; i.int, inner integument; o.int, outer integument.
654. If we cut across the compound pistil of trillium we find that the infoldings of the three pistils meet to form three partial partitions which extend nearly to the center, dividing off three spaces. In these spaces are the ovules which are attached to the infolded margins. If we make cross-sections of a pistil of the May-apple (podophyllum) and through the ovules when they are quite young, we shall find that the ovule has a structure like that shown in [fig. 389]. At m is a cell much larger than the surrounding ones. This is called the macrospore. The tissue surrounding it is called here the nucellus, but because it contains the macrospore it must be the macrosporangium. The two coats or integuments of the ovule are yet short and have not grown out over the end of the nucellus. This macrospore increases in size, forming first a cavity or sac in the nucellus, the embryo sac. The nucleus divides several times until eight are formed, four in the micropylar end of the embryo sac and four in the opposite end. In some plants it has been found that one nucleus from each group of four moves toward the middle of the embryo sac. Here they fuse together to form one nucleus, the endosperm nucleus or definitive nucleus shown in [fig. 390]. One of the nuclei at the micropylar end is the egg, while the two smaller ones nearer the end are the synergids. The egg-cell is all that remains of the archegonium in this reduced prothallium. The three nuclei at the lower end are the antipodal cells.
Fig. 390.
Podophyllum peltatum, ovule containing mature embryo sac; two synergids, and eggs at left, endosperm nucleus in center, three antipodal cells at right.
Fig. 391.
Macrospore
(one-celled stage)
of lilium.