. A practical course in botany, with especial reference to its bearings on agriculture, economics, and sanitation. Botany. 356 PRACTICAL COURSE IN BOTANY In the gymnosperms, — pines, yews, cycads, etc., — which represent the most ancient and primitive type of existing seed-bearing plants, the similarity of these processes to those of certain of the pterido- phytes is very striking, and it was through the study of these that the sequences of the process were traced in the much more obscure form in which they occur among the angi- osperms. From the endosperm in the seeds of gymnosperms arche- go
. A practical course in botany, with especial reference to its bearings on agriculture, economics, and sanitation. Botany. 356 PRACTICAL COURSE IN BOTANY In the gymnosperms, — pines, yews, cycads, etc., — which represent the most ancient and primitive type of existing seed-bearing plants, the similarity of these processes to those of certain of the pterido- phytes is very striking, and it was through the study of these that the sequences of the process were traced in the much more obscure form in which they occur among the angi- osperms. From the endosperm in the seeds of gymnosperms arche- gonia were found to be developed (Fig. 510) in much the same way as Fig. 510. — Diagrammatic section through the in Selaginella from the ovule of a gymnosperm belonging to the spruce ' family: i, integument covering the ovule ; e, endo- prothallium, tnUS sperm (corresponding to female gametophyte), gj^owins the endo- whieh fills the embryo sac, containing two arche- ° gonia, a; o, egg cell; p, pollen grains; t, pollen sperm to be a modified tubes entering the neck, c, of the archegonia. j , i j j gametophyte. In some cases, it has even been found to protrude a little way out of the embryo sac and to take on a slightly greenish tinge — another renrniscence of its origin. Fertilization, too, takes place in precisely the same manner as in the pteridopbytes, except that in all but the ginkgo and the cycads, the fertilizing cells in the pollen grains are non-motile, and find their way to the ovule by growing down into the embryo sac with the pollen tube, instead of swimming to it — an adaptation probably brought' about in response to changed conditions during the course of evolution from aquatic to terrestrial Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Andrews, Eliza Frances, b. 1840; Lloyd,
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