906 PHENOMENA OF SEXUAL REPRODUCTION. 



Hildebrand, and others ^ In spite of the hermaphrodite flowers of Phanerogams 

 and the similar sexual arrangements 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 when the sexual organs are contiguous. 



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, 

 when they are contiguous, are mature at different times, so that the sexual cells 

 which are in close contiguity and are therefore nearly related are not capable of 

 performing their respective functions simultaneously. The male cell must in these 

 cases unite with a female cell in a different group of sexual organs. This is in fact 

 usually the case with the hermaphrodite flowers of Angiosperms, as also with most 

 prothallia of Ferns'and the monoecious Characese, in which the carpogonium is situated 

 close to the antheridium but becomes mature only at a later period (this is very 

 strikingly the case in Nitella flexilis). Insects are the main agents 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 

 presently. 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 carpogonia on other leaves of the same plant, or even 

 on other plants of the same species. Whether the Algae named above and some 

 Muscineae 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, 

 there are also other contrivances of a very different nature which have the sole 

 purpose of transferring the pollen of hermaphrodite flowers, by the help of insects, 

 to the stigma of another flower of the same or of a different plant. In most 

 Orchidese, Asclepiadese, Viola, &c., the reproductive organs of each individual flower 

 are developed at the same time, but at the time of maturity mechanical contrivances 

 exist which prevent the pollen falling on the stigma of the same flower ; it must be 

 carried by insects to other flowers. 



In other cases, as Hildebrand has shown in the case of Corydalis cava"^, the 

 pollen does actually fall on the stigma of the same flower, but is there impotent, 

 having the power of fertilising only when it falls on the stigma of a different flower, 

 and only perfectly when carried to the flower of a different individual of the same 

 species. Such a plant is therefore only morphologically hermaphrodite; it is 



^ K. C. Sprengel (Das neu entdeckte Geheimniss der Natur im Bau und in der Befruchtung 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 Contrivances 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). — See Darwin, The Effects of Cross- and 

 Self-Fertilisation in the Vegetable Kingdom, 1876.] 



^ [Ueber die Befruchtung von Corydalis cava, Jahrb. fiir wiss. Bot. 1866.] 



