Fig. 55.—“Seed”
In appearance this is like a seed, but differs from a true seed in having no embryo, and is like the preceding structure in having a very large spore, S, though there is no trace of the three aborting ones. The spore develops in a special mass of tissue known as the nucellus, n, which partly corresponds to the sporangium wall of the previous types. In it a cavity, p c, the pollen chamber, receives the pollen grains which enter at the apex of the “seed”. There is a complex coat, C, which stands round the nucellus but is not joined to it, leaving the space l between them. Only in fossils; Trigonocarpus (see [p. 122]) is similarly organized. Small spores in fern-like sporangia, called pollen grains.
Fig. 56.—“Seed”
Very similarly organized to the above, but the coat is joined to the nucellus about two-thirds of its extent, and up to the level l. In the pollen chamber, p c, a cone of nucellar tissue projects, and the upper part of the coat is fluted, but these complexities are not of primary importance. The large spore S germinated and was fertilized within the “seed”, but apparently produced no embryo before it ripened. Small “spores” in fern-like sporangia form the pollen grains. Only in fossils, e.g. Lagenostoma. (See [p. 119].)
Fig. 57.—Seed
Essentially similar to the preceding, except in the possession of an embryo e, which is, however, small in comparison with the endosperm which fills the spore S. The whole organization is simpler than in the fossil Lagenostoma, but the coat is fused to the nucellus further up (see l). Small “spores” form the pollen grains. Living and fossil type, Cycads and Ginkgo.
Fig. 58.—Seed