Файл:Evolution of plants (1911) (14584806450).jpg

Identifier: evolutionofplant00scot (find matches)
Title: Evolution of plants
Year: 1911 (1910s)
Authors: Scott, Dukinfield Henry
Subjects: Plants--Evolution.
Publisher: New York: H. Holt and company (etc., etc.
Contributing Library: University of Florida, George A. Smathers Libraries
Digitizing Sponsor: University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation

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o the whole family. Itmay be added that the very first Seed-plant inwhich spermatozoids were found was not a Cycad,but the Maidenhair-tree (Ginkgo), where theywere observed by Hirase a month or two beforetheir discovery in Cycax by Ikeno. A short ac-count of the process as it has been observed inCycas and Zamia may be given before we furtherconsider the significance of the facts. The ovule, at the time when it is ready to re-ceive the pollen, is about the size of a grain ofMaize, or a small Hazel-nut. It consists of anouter envelope and a central body, the two beingclosely united except at the top, where a narrowpassage (the micropyle) is left open in the envel-ope, leading down to the central body, or nucellus,as it is technically called (fig. 7, d). The apex ofthe nucellus becomes excavated to form a deep pit,the pollen-chamber, a character ony shown by theCycads and the Maidenhair-tree among livingplants, though common in Palaeozoic seeds (seeChapter IV). 72 THE EVOLUTION OF PLANTS
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Fig. 7.—(A) Pollen-tube of Microcycas, containing numerous male cells X 90; (B) end of pollen-tube of Zamia, containing the two ciliated spermatozoids; (C) a free spermatozoid, showing the spiral band bearing the cilia (B and C X 80);(D) longitudinal section of ovule of Cycas, showing embryo-sac, nucellus with pollen-chamber, and integument slightly magnified. A after Caldwell; B and C after Webber; D after Griffith. THE EVIDENCE 73 The pollen, whether blown by the wind, or, as seems likely to be the case in some South African Cycads, conveyed by insects, is received in the micropyle by a drop of gummy substance, in which the pollen-grains stick. As the drop evaporates or is re-absorbed, the pollen-grains are drawn down through the narrow passage into the pollen-chamber below (fig. 7, d). Here each grain anchors itself by sending out a branched tube into the surrounding tissue. Thus pollination is effected; the actual fertilisation does not take place for some months later, when the ovule has grow

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