8o8 PHENOMENA OF SEXUAL REPRODUCTION. 



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 

 Fiicus plaiycarpiis produces both oospheres and spermatozoids ; the nucules of most 

 Characeae 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 others'. 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 

 DicJiogamy, 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 widi most 

 prothallia of Ferns and the monoecious Characeae, where the nucule is situated close 

 to the globule but becomes mature only at a later period (this is very strikingly the 

 case in Niiella flexilis). Insects are the main agent in the conveyance of the poUen 

 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- 

 cineae 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, 



^ K.C. Sprengel (Das neu entdeckte Geheimniss der Natur im Bau und in der Befruchtimg der 

 Blumen, Berlin, 1 793, 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 P^ertilised, 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.] 



