REPRODUCTION AND DISPERSAL 863 



In composites with conical or columnar receptacles (such as Rudbeckia and 

 Lepachys), there occurs what may be termed gravity pollination, pollen from the 

 upper flowers dropping upon the stigmas of the lower and older flowers. In An- 

 themis, as the flowers develop, the disk elongates in such a way that the stigmas of 

 the older flowers are exactly under the shedding stamens of the younger flowers. It 

 will be recalled that in those composites that tend toward dicliny, it is the outer 

 (older) flowers that ordinarily are pistillate, and the inner (younger) flowers that 

 ordinarily are starainate. So far as geitonogamy is concerned, such a condition is 

 economical, since stamens would be useless in the outer flowers and pistils in the 

 inner flowers, though they might be of value in case of cross pollination by insects. 



The structural facilitation of geitonogamy in the umbeUifers is almost as marked 

 as in the composites. In Eryngium the flowers are in dense heads that facilitate 

 contact pollination between adjoining flowers. In Sanicula there are monoclinous 

 and staminate flowers, the long styles of the former bending over and bringing the 

 stigmatic surfaces into contact with the latter. In such wind-pollinated composites 

 as Ambrosia, geitonogamy is likely to take place between flowers of different heads, 

 as the staminate heads are uppermost. 



Close pollination or autogamy. — General remarks. — Autogamy, 

 that is, pollination between anthers and stigmas of the same flower, once 

 thought to be relatively rare, is now known to be extremely common. In 

 some cases the features that facilitate autogamy are quite as striking 

 as are those previously mentioned features that impede or prevent it. 

 Autogamy by contact may be called self pollination, a term often incor- 

 rectly made synonymous with autogamy in general ; as with geitonogamy, 

 close pollination occurs chiefly by contact or through the agency of grav- 

 ity, though wind and insects also may be operative. In many cases, 

 especially where foreign pollen is prepotent over own pollen, autogamy 

 probably is effective only in the absence of cross pollination; in many 

 other cases both appear to be equally efifective, and in a number of in- 

 stances autogamy is the only form of pollination possible. Careful 

 study has shown that in alpine and arctic regions autogamy habitually 

 exceeds xenogamy. 



Illustrations of autogamy. — Simple cases of autogamy occur in Trillium and in 

 Gerattium, the anthers and stigmas being in close juxtaposition; most such flowers 

 are slightly protogynous, so that cross pollination may occur before there is a chance 

 for contact pollination. Somewhat more complex are the many cases in which 

 anthers and stigmas come into contact through growth movements, as in the stamen 

 movements of Circaea and of many crucifers, and in the style movements of Epilo- 

 bium and of many other plants; such flowers also are as a rule slightly dichogamous. 

 In the composites autogamy as well as geitonogamy may take place through style 

 inflections and through the closing and the opening of heads; similarly, many flowers 

 that open and close daily may exhibit autogamy (as in Gentiana). Pollen drops 

 directly on the stigma, illustrating gravity pollination in many erect flowers with 



