Text-book of botany : morphological and physiological / by Julius Sachs ; translated and annotated by Alfred W. Bennett ; assisted by W.T. Thiselton Dyer.

Date:
1875
Catalogue details

Licence: Public Domain Mark

Credit: Text-book of botany : morphological and physiological / by Julius Sachs ; translated and annotated by Alfred W. Bennett ; assisted by W.T. Thiselton Dyer. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    of near even if not always of the closest affinity. Thus, for example, the same cellular filament of (Edogonium produces both male and female cells, the same Vaucheria-tube antheridia and oogonia in close proximity, the same conceptacle of Fucus plaiycarpus produces both oospheres and spermatozoids ; the nucules of most Characese are produced close beside the globule on the same leaf; the archegonia and antheridia of some Mosses (species of Bryum) are collected together in herma- phrodite ‘flowers’; the prothallia of many Ferns produce both kinds of reproductive organs side by side; in the flowers of Angiosperms hermaphroditism is the typical and most common arrangement. But in all these cases where the aim is apparently to favour the union of sexual cells nearly related to one another, there are at the same time contrivances which hinder the male cells from reaching the contiguous female cells ; or at least to render it possible that this should not always happen. This fact was first recognised by Kolreuter (1761) and Karl Conrad Sprengel (1793), and has been further illustrated recently by Darwin, Hildebrand, and others1. In spite of the hermaphrodite flowers of Phanerogams and the similar sexual arrange- ments of Cryptogams, it appears very certain that the union of nearly related sexual cells must be unfavourable to the perpetuation of most plants, since such various and often astonishing means are provided in order to prevent self-fertilisation within a hermaphrodite flower. One of the simplest and commonest means for ensuring cross-fertilisation is Dichogamy, i. e. the arrangement by which the two kinds of reproductive organs, even when contained within the same flower, are mature at different times, so that the sexual cells which are in close contiguity, and therefore nearly related, are not capable of performing their respective functions simultaneously. The male cell must in these cases unite with the female cell from a different flower. This is in fact usually the case with the hermaphrodite flowers of Angiosperms, as also with most prothallia of Ferns and the monoecious Characese, where the nucule is situated close to the globule but becomes mature only at a later period (this is very strikingly the case in Nilella flexilis). Insects are the main agent in the conveyance of the pollen to the stigma of other flowers of dichogamous Phanerogams, for which purpose the parts of the flower possess special adaptations which will be described pre- sently. In the dichogamous species of Nitella and prothallia of Ferns the motility of the antherozoids is sufficient to enable them to reach the archegonia of neigh- bouring prothallia, or the nucules on other leaves of the same plant, or even on other plants of the same species. Whether the Algae named above and some Mus- cinese are dichogamous is doubtful; but the motility of the antherozoids renders it possible for them to reach the oospheres of other plants or those on other branches of the same plant. Among Angiosperms, in addition to the common occurrence of dichogamy, 1 K. C. Sprengel (Das neu entdeckte Geheimniss der Natur im Bau und in der Befruchtung der Blumen, Berlin, 1793, p. 43) first gave expression to the pregnant idea, ‘ Since a large number of flowers are diclinous and probably at least as many hermaphrodite flowers are dichogamous, Nature appears to have designed that no flower shall be fertilised by its own pollen.’ Darwin (On the Various Con- trivances by which Orchids are Fertilised, London, 1862, p. 359) says, ‘Nature tells us in the most emphatic manner that she abhors perpetual self-fertilisation and again, ‘ No hermaphrodite fertilises itself for a perpetuity of generations.’ [This last observation was first made by Andrew Knight in 1799 (Phil. Trans, p. 202).—Ed.]