42 INTRODUCTION 



in the evening. Among Tubuliflorae, Geitonogamy is comparatively rare. In 

 Eupatorium cannabinum (see Fig. 2), and in other species of this genus, the branches 

 of the stigmas of the older florets of a head separate so widely that they touch 

 the pollen-covered hairs of the neighbouring florets, which are just beginning to 

 open. In Tussilago, when the head closes in the evening, the pollen emptied 

 out of the anther-tubes of the disk-florets is transferred to the ligulate ray-florets. 

 When the head opens again, the pollen slides down the ligulate corollas to the 

 stigmas situated at their bases. In Calendula, the style-branches of the ligulate 

 ray-florets curve over the pollen-covered disk-florets, and thus get pollinated. In 

 the Asterineae (Aster, Solidago, &c.) the pollen of the disk-florets falls on to the 

 already mature stigmas of the ray-florets, in consequence of the inclination of 

 the heads. In many Compositae (Homogyne, Artemisia, Doronicum, Senecio, 

 Telekia, Buphthalmum, Anthemis, Matricaria, and others) the branches of the stigmas 

 curve back like a bow, so that the pollen clinging to the hairs of the style reaches 

 the stigmatic surfaces of the older adjacent florets. This is rendered particularly 

 easy in these flowers by the fact that the torus is arched, so that the inner and 

 younger florets (which are covered with pollen) are necessarily at a higher level than 

 the outer and older ones, which are already in the female condition. 



Among Umbelliferae the arrangements leading to Geitonogamy are scarcely 

 less varied than in Compositae. Kemer (op. cit., pp. 323-5) describes the arrange- 

 ments in question present in Eryngium, Astrantia, Sanicula, Laserpitium, Pachy- 

 pleurum, Siler, Athamantha, Meum, Chaerophyllum (see Fig. 2), Anthriscus, 

 Foeniculum, Coriandrum, Slum, Ferulago. Kerner further mentions that these may 

 serve as types for numerous plants of other families, of which the flowers are 

 crowded together in heads, clusters, corymbs, spikes, or racemes ; especially among 

 the Cornaceae, Caprifoliaceae, Rubiaceae, Scrophulariaceae, Rosaceae, Polygonaceae, 

 Liliaceae, Aroideae. He subsequently describes the Geitonogamy of Calla palustris, 

 Saxifraga juniperifolia; Veronica maritima, spicata, and spuria ; Eremurus caucasicus, 

 Allium victorialis. Polygonum Bistorta, Rheum, Rumex alpinus ; Thalictrum alpinum. 

 foetidum, and minus ; Erica carnea, Lathraea squamaria, Clandestina rectiflora, 

 Bartsia alpina, Crucianella stylosa. These relations are briefly described in the 

 second volume of my work in dealing with floral adaptations, so far as the plants 

 named by Kerner belong to the European Flora. 



IV. Xenogamy. 



Reference has already been made in the historical part to the significance of 

 Xenogamy, which was first recognized by Darwin, as well as to the modification 

 by Hermann Miiller of the resultant Knight-Darwin law. It occurs not only among 

 diclinous plants, but also in most monoclinous plants, since, in the latter, male 

 and female floral stages are brought about by Dichogamy. As already mentioned 

 in the synopsis of floral arrangements (p. 29) only cross-pollination is possible 

 in such cases, for as stamens and carpels mature at different times, the stigmas have 

 shrivelled up by the time the anthers dehisce, or vice versa. In plants where 

 Dichogamy is not so well marked cross-pollination predominates at first, but self- 

 pollination later on becomes possible. 



