How Plants Work by Stephen Blackmore

Author:Stephen Blackmore
Language: eng
Format: epub
Publisher: Quarto Publishing Group
Published: 2018-02-26T16:00:00+00:00

In the fossil genus Caytonia, cupules were produced on an elongate, simply branched axis (A). Each cupule was curved so that its opening faced back toward its base (B). Within each cupule, ovules were lined up along its ‘backbone’, with their openings facing the opening of the cupule (C). Pollen sacs were grouped in fours (D) and borne on branching structures (E).

Cupule conundrums

In many Mesozoic gymnosperms, ovules were protected in structures called cupules. In many ways these functioned like carpels, most likely protecting ovules from desiccation and/or herbivorous insects. As in the earlier evolution of the integument around ancient megasporangia (see here), cupules appear to have evolved from leaf segments that wrapped around clusters of ovules. In all known fossil cupules, the ovules are straight and have only one integument, as in other gymnosperms. The characteristic ovule of angiosperms, however, is bent backwards on its axis (anatropous) and has two integuments. Ovules, in turn, are surrounded by the carpel. So carpels most likely did not evolve directly from the cupules of any known gymnosperms, otherwise angiosperm ovules would still be straight and missing the second integument (see box opposite).

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