793 



LECTURE XLV. 



come in as additional aids. These movements are often of such a kind that at i 

 certain time the opened anthers assume the same position in the flower which 

 the stigmas in a receptive condition have at another time, so that the insect, by 

 the same movement, brings the same part of its body in the one flower into con- 

 tact with the opened anthers, in the other with the receptive stigmas, and 

 wipes off on to the latter the pollen which hangs on to that part of its body. The 

 same principle is also employed in the case of heterostylous flowers, in so far that 

 here the pollination is most successful .when anthers and stigmas which occupy 

 the same (permanent) position in the different flowers, co-operate with the aid of 

 insects. 



In addition, however, there are most manifold, often perfectly astounding 

 mechanical adaptations to ensure the transference of the pollen by the aid of 

 insects. A few examples may now be cited in illustration. 



(i) Dichogamous plants are either protandrous or protogynous. In the 

 former the stamens are developed first, and their anthers dehisce at a time when 

 the stigmas are still undeveloped and not yet receptive for pollination : the 

 stigmatic surfaces do not open until later, mostly not until the pollen from the 

 anthers of the same flower has been borne away by insects, so that they can 

 only then be pollinated by the pollen of younger flowers. This is true of species 

 of Geranium, Pelargonium, EpiloUum, Malva, the Umbelliferse, Compositae, 

 Campanulacese, Lobeliaceae, Digitalis and others. The observation of these 

 points, and also of the movements of stamens and stigmas mentioned above, is 

 so easily accompKshed in these cases, e. g. Geranium, AlthcEa, that a detailed 

 description scarcely appears necessary. In the case of protogynous dichogamous 

 flowers, the stigma is receptive at a time when the anthers of the same flower 

 are not yet ripe; when these dehisce later, and allow the pollen to escape, the 

 stigma has been already pollinated by pollen from a distance, or has even already 

 withered and fallen off (e. g. Parietaria diffusa). The pollen of this flower can 

 therefore only be employed for younger flowers ; this is the case in Scrophularia 

 nodosa, Mandragora vernalis, Scopolia atropoides, Plantago media, Luzula pilosa, 

 Anthoxanthum odoratum, and others (Hildebrand). Among protogynous dicho- 

 gamous flowers Aristolochia Clematitis is distinguished by specially conspicuous 

 and peculiar adaptations. 



Fig. 45 1 .<4 shows a young flower in longitudinal section ; the stigmatic 

 surface n is just ready for fertilisation, but the anthers are still closed. A small 

 fly i, which has brought on its back a heap of pollen from an older flower, has 

 just penetrated through the narrow throat of the flower, and is roaming about 

 in the flask-like enlargement k. Not uncommonly six to ten such ilies may be found 

 in a flower : they are imprisoned and cannot escape, because the throat r of the 

 flower is beset hke a trap with long motile hairs, which ofi'er no hindrance to 

 the entrance of the flies, but bar the passage out as in a weir-basket. While the 

 insect is thus wandering around the cavity, it brings its pollen-laden back in con- 

 tact with the stigmatic surface, and pollinates it, in consequence of which the 

 stigmatic lobes curve upwards, as in Fig. 451 B n. As soon as this has taken 

 place, the anthers, which have been closed hitherto, dehisce, and become freely 

 accessible, at the same time, by the change in the stigma, and by the collapse 



