8lO PHENOMENA OF SEXUAL REPRODUCTION. 



While in the very numerous diclinous, dichogamous, dimorphic, and trimorphic 

 flowers, insects carry pollen from one flower to another, it is comparatively rare for 

 cross-pollination to take place without the help of insects. This occurs in some 

 Urticaceae, as Pilea and Broussonetia, where the anthers emerge suddenly from the 

 bud and scatter their light pollen in the air like a fine cloud of dust, which is then 

 blown to the female organs of other flowers. In the rye the arrangement is still 

 simpler ; the flowers open separately, usually in the morning ; the filaments elongate 

 rapidly and push the ripe anthers out of the pales ; the anthers then hang down at 

 the end of the long filaments, open, and allow the heavy pollen to fall down, thus 

 reaching the stigmas of other flowers lower down in the same spike or in neigh- 

 bouring spikes, being assisted in this by the oscillations of the haulm under the 

 influence of the wind\ 



In connection with the tendency so clearly evidenced even among Cryptogams, 

 and still more among Phanerogams, to prevent self-fertilisation within the same 

 hermaphrodite flower, it is a very remarkable fact that there are a number of plants 

 among Angiosperms which form two kinds of hermaphrodite flowers, viz. large 

 flowers which can generally be fertilised by the pollen of other flowers, and small, 

 more or less depauperated flowers, sometimes underground, which never open [C/ei- 

 stogamous Flowers], the pollen emitting its tubes immediately from the anthers and 

 thus fertilising the ovules. There occur therefore in these cases different kinds of 

 flowers on the same individual, one kind being adapted for cross-, the other kind 

 exclusively for self-fertilisation^. This occurs, for example, in Oxalis Acetosella, 

 where the small flowers are formed close to the ground when the larger flowers have 

 already ripened their fruit ; in Impaiiens Noli-me-tangere, Lainiwn amplexicaule, 

 Speciilaria perfoliata, many species of Viola, as V. odorata, elatior, caniiia, mirahilis^ 

 &c., Rtiellia clatidesima, many Papilionaceae, as Amphicarpsea, and Voandzeia, Co?n- 

 melyna bengalemis, &c. When in these cases the large typically developed flowers 

 are fertile, cross-ferdlisation with other flowers of the same species must happen 

 occasionally in the course of generations, and the small depauperated self-fertilised 

 flowers then seem to be a subsidiary contrivance whose purpose is altogether un- 

 known. It is however remarkable, and apparently in contradiction to the general 

 rule, that the large normal flowers sometimes exhibit a tendency to infertility (as in 

 species of Viola) or are altogether unfruitful (as in Voandzeia), so that reproduction 

 depends in such cases mainly or entirely on the cleistogamous self-fertilised flowers. 

 But since there are many questions in connection with this subject that are not yet 

 solved, these rare exceptions cannot overthrow the general law^ 



' [For a detailed account of the very remarkable phenomena connected with the pollination of 

 rye and other cereals, see Hildebrand in Gardener's Chronicle, March 15 and 22, and May 24, 1873 ; 

 also A. S. Wilson, Trans. Bot. Soc. Edin. XI, 506 and XII, 84. Flowers the pollination of Avhich 

 is effected by the wind are termed aneinophilous, in contradistinction to the e?itomophilo7is, or those 

 pollinated by the agency of insects. — Ed.] 



2 H. V. Mohl, Einige Beobachtungen iiber dimorphe Bliithen, Bot, Zeit. 1863, Nos. 42, 43. [See 

 also A.W.Bennett on the closed self-fertilised flowers of Impatiens in JomTi. Linn. Soc. 1872, 

 p. 147; ditto. Pop. Sci. Rev. 1873, p. 337. In Juncjis htifojiius the pollen-tubes are emitted while 

 the pollen-grains are still enclosed in the anther, perforating the wall of the latter. — Ed.] 



^ [Herrmann Miiller (Nature, vol. VIII, p. 433 et seq.) has pointed out the existence of another 

 kind of dimorphism, in which a species presents two different forms of flowers, one adapted to self-i 



