During recent years a number of fossil (Carboniferous and Permian) plants have been very thoroughly investigated in the light of modern anatomical knowledge, and as a result it has become st i s clear that in those times a large series of plants etisted ear ys intermediate in structure between the modern ferns tern of Cycaand the modern Gymnosperms (especially Cycads), dofiices.
Among the Cycadofilices a series of stages is found leading from the primitive fern-protostele to the type of siphonostele characteristic of the Cycads which agrees in essentials in all the Spermophytes.
The leaves of the more primitive members of this series were entirely fern-like and possessed a fern-like vascular strand; while in the later members, including the modern Cycads, the leaf bundles, remaining unaffected by secondary thickening, are mesarch, while those of the stem-stele have become endarch.
Among the existing Cycads, though the type of vascular system conforms on the whole with that of the other existing seed-plants, peculiar structures are often found (e.g.
In Monoblepliaris, one of the lower Fungi, in some Algae, in the Vascular Cryptograms, in Cycads (Zamia and Cycas), and in Ginkgo, an isolated genus of Gymnosperms, the male cell is a motile spermatozoid with two or more cilia.
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