864 ECOLOGY 



styles shorter than the stamens (as in the lilac), and in many pendulous flowers with 

 styles longer than the stamens (as in Dodecatheori). In Pedicularis the growing 

 corolla rubs over the anthers, so that the pollen falls upon the stigma underneath. 

 In Cyclamen and Moneses the growth curvatures of the flower stalk at the time of 

 anther maturity cause the pollen to drop out upon the stigma. 



Yucca. The flowers of Yucca commonly are pendent (fig. 1192), and though 

 the stigmas extend beyond the anthers, pollination by gravity is unlikely if not im- 

 possible. The flowers are nocturnal, blossoming but once, and are pollinated by 

 a small moth, Pronuba. The females pierce the ovaries with their ovipositors and 

 lay eggs among the ovules, before and after which they deliberately take pollen 

 from the anthers, holding it in their specially constructed maxillary palps, and ram 

 it into the stigma. As a result of this astounding process the ovules develop; each 

 larva eats about twenty, and the rest develop into seeds. This symbiosis is be- 

 lieved to be obligate for each symbiont; in any event, Yucca is seedless in the ab- 

 sence of Pronuba. The mode of evolution of an instinct that impels an insect to 

 stuff a stigma with pollen cannot even be imagined. 1 



Cleistogamy. The culmination of structures facilitating close pol- 

 lination is found in flowers that never open, since in these with rare ex- 

 ceptions autogamy alone is possible. Flowers that regularly open, such 

 as those heretofore considered, are termed chasmogamous, while flowers 

 that never open are called deistogamous. Cleistogamy may be habitual 

 (as in the subterranean flowers of Viola and Poly gala), or it may be 

 facultative, depending upon definite external factors. Conspicuous cases 

 of facultative cleistogamy are found in Oxalis, Specularia, Impatiens, 

 and Lamium. Subjection to low temperature is believed to be the chief 

 cause of such cleistogamy; in Lamium, for example, the spring and 

 autumn flowers are cleistogamous, while the summer flowers are chas- 

 mogamous or sometimes cleistogamous in cold or rainy weather. In 

 Viola sepincola the aerial flowers are chasmogamous in the sunshine and 

 cleistogamous in the shade, indicating that light as well as heat may be 

 a factor. The submersed flowers of Alisma also are cleistogamous. 



Habitual cleistogamy is well illustrated by the subterranean and 

 generally colorless flowers of Viola cucullata (and many other violets), 

 Amphicarpaea, and Poly gala polygama (fig. 1191); in Viola cucullata 

 they appear some weeks or months after the showy aerial flowers, but 

 in Polygala the two kinds of flowers are nearly synchronous. In all 

 three cases they are much more productive than are the showy and pre- 

 sumably cross-pollinated aerial flowers. In the rock rose (Helianthe- 



1 While Yucca is here considered as illustrating close pollination, the moths, after 

 gathering pollen, often fly to other flowers or even to other plants, so that they may effect 

 autogamy, geitonogamy, or xenogamy. 



