INFLUENCE OF THE ORIGIN OF CELLS ON FERTILISATION. 813 



usually not till the pollen has been carried away from the anthers by insects ; they can 

 then only be fertilised by the pollen of younger flowers. To this category belong the 

 various species of Geranium, Pelargonium, Epilobium, Malva, Umbelliferae, Compositae, 

 Campanulaceae, Labiatae, Digitalis, &;c. The phenomena referred to, especially the 

 movements of the stamens and stigmas, are so readily observed in these cases, e.g. in 

 Geranium and Althaea, that no further description is necessary. In protogynous flowers 

 the stigma is receptive before the anthers in the same flower are mature ; when these 

 subsequently open and allow the pollen to escape, the stigma has already been pollinated 

 by foreign pollen or has even withered up and fallen off" (as in Parietaria diffusa) ; and 

 the pollen of these flowers can therefore only be apphed to the fertilisation of younger 

 flowers. To this class belong Scrophularia nodosa^ Mandragora 'vernalis, Scopolia atropoides, 

 Plantago media, Luzula pilosa, Ant box ant hum odoratum, &c. Among protogynous flowers 

 Aristolochia Clematitis is characterised by striking and peculiar contrivances. 



In Fig. 458 A is shown a young flower cut through lengthwise; the stigmatic sur- 

 face n is already in a receptive condition, but the anthers are still closed ; a small 

 fly /, which has brought on its back a mass of pollen from an older flower, is forced in 

 through the narrow throat of the perianth, and runs about in the globular swelling k ; 

 as many as from six to ten flies are not unfrequently found in one flower. They are shut 

 up and cannot escape, because the throat of the perianth r is furnished with long hairs 

 moving as on a hinge, which present no impediment to the entrance of the insect, but pre- 

 vent its escape like a trap. While the insect is moving about in the cavity, its back laden 

 with pollen comes into contact with the stigmatic surface and pollinates it, in conse- 

 quence of which the lobes of the stigma curve upwards, as is shown in Fig. 458 5, «. As 

 soon as this has taken place, the anthers, previously closed, open ; they are laid bare by 

 the change in the position of the stigmas, and are rendered accessible by the withering up 

 of the hairs at the bottom of the cavity of the flower, which has now become wider. The 

 flies which have now carried their pollen on to the stigmatic surface, can therefore creep 

 down to the open anthers, where the pollen again becomes attached to them. By this 

 time the throat of the perianth r has again become passable, the net-work of hairs in it 

 having died and withered away after the pollination of the stigma. The insect, laden 

 with the pollen of this flower, can now escape, and again performs the same work in 

 another flower. But while the changes which have been described are taking place 

 inside the flower, its position has also altered. As long as the stigma is still receptive, 

 the pedicel is erect and the perianth open outwards (Fig. 457 //), so that the visiting flies 

 find a door hospitably open. But as soon as the pollination of the stigma has been 

 eff"ected, the pedicel bends sharply downwards just beneath the ovary, and when the 

 flies, again laden with pollen, have flown out of the flower, the standard-like lobe of the 

 perianth above the mouth of the tube (Fig. 458 5) closes, preventing the entrance of 

 the flies, whose visits would now be useless. 



(2) Floivers in <which the anthers and stigmas are mature at the same time, but self- 

 fertilisation is hindered or prenjented by the position of the organs and by mechanical contri- 

 'vances. The pollen is in these cases also usually carried to the stigma by insects, but 

 generally in such a manner that the stigma can only be pollinated by the pollen from 

 another flower, though sometimes, as in Asclepiadeae, pollination from the same flower 

 is not impossible in addition to cross-fertilisation. The contrivances for this purpose 

 are astonishingly numerous, and sometimes so complicated that their purpose can only 

 be detected by very careful investigation. To this category belong, for example, the 

 various species of Iris, Crocus, and Pedicularis, many Labiatae, Melastomaceae, Passiflor- 

 ace^, and Papilionace^. Among the most interesting examples are the Asclepiadeae, 

 in which however the contrivances could be explained only by lengthy descriptions and 

 a large number of illustrations \ In Sal-via pratensis and some other species of this genus 



^ For a fuller description, see R. Brown, Observations on the Organs and Mode of Fecundation 

 in Orchide^ and Asclepiadece ; Trans, Linn. Soc. 1833, and Hildebrand in Bot. Zeit. 1867, No. 34- 



