File:The natural history of plants, their forms, growth, reproduction, and distribution; (1902) (14763385004).jpg

Identifier: naturalhistoryof02kern (find matches)
Title: The natural history of plants, their forms, growth, reproduction, and distribution;
Year: 1902 (1900s)
Authors: Kerner von Marilaun, Anton, 1831-1898 Oliver, Francis Wall, 1864- Macdonald, Mary Frances Ewart Busk, Marian Balfour, Lady
Subjects: Botany
Publisher: London, Blackie
Contributing Library: NCSU Libraries
Digitizing Sponsor: NCSU Libraries

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at another sexual organs. This brief summary is sufficient to showthat a given Vaucheria-pl&nt has no inherent tendency to reproduce asexually inpreference to sexually, or conversely; and that its manner of reproduction (or itsabstention from reproduction) depends on the conditions which prevail outside theplant. Thus, in Vaucheria, no true alternation of generations prevails in the sensein which it does in Mosses and Ferns, and every generation is potentially both asexual and an asexual generation. It is the external conditions which call forththe one or the other. In a great many other Thallophytes the same is no doubt true, though exactexperiments have yet to be performed on the majority of them. We know it tobe so in Botrydium and in the Water-net (Hydrodictyon) and in others. TheWater-net (figured on p. 24) propagates asexually by the contents of its cellsbreaking up into very numerous (7000-20,000) swarm-spores (thallidia) which HETEEOMORPHISM AND ALTERNAT.OM OP GEKERATION..
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Fig. Jj4 —Larcli-tieei (Latix euiopcea). 484 HETEROMORPHISM AND ALTERNATION OF GENERATIONS. do not forthwith escape, but swim about for a while within the wall of thecell in which they originate. Then they join together into a tiny net whichescapes, ultimately growing to its full size (c/. vol. i. p. 36). In sexual repro-duction a much larger number (30,000-100,000) of small motile bodies (gametes)escape and conjugate in pairs. Klebs has found here also that either of thesemethods can be produced at will by altering the conditions under which the plantgrows. Among the Fungi the Saprolegniacese show well-marked sexual and asexualmethods of propagation. These are mould-like forms which attack fish and otheraquatic animals upon which they are parasitic. Purely asexual reproduction occursby means of zoospores which are liberated from long, club-shaped sporangia (fig.352^); whilst sexual reproduction (which may occur upon the same plant) takesplace by spherical oogonia arising upon certa

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