516 



BOTANY 



Entomophilous Flowers 



Dichogamy. In the majority of the Angiosperms the stamens and 

 carpels are borne together in the same flower, and yet it is found 

 that, as a rule, the pistil is not pollinated from the stamens of the 

 same flower. The simplest device by which self-pollination is pre- 

 vented is the maturing of the stamens and carpels at different times 

 (Dichogamy). In case the stamens mature first, it is known as 

 Proterandry ; if the pistils are first mature, Proterogyny. The former 

 is very common, the latter less so. Examples of proterandry are 

 offered by the various forms of garden Geraniums (Pelargonium) and 

 Nasturtium (Tropseolum). One of the commonest examples of pro- 

 terogyny is the common Plantain (Plantago 

 major) (Fig. 483). The younger flowers of 

 k ~\>&2f a spike show the stigmas protruding before 



H n)/^~^ ^ e stamens discharge their pollen. In the 



older flowers, where the stigma has already 

 been pollinated, the stamens are conspicuous. 

 Many Araceae are also proterogynous. 



Unless the flowers are close together, as 

 in the Plantain, so that pollen from an older 

 flower may be easily shaken upon or blown 

 to a younger flower, some special agent must 

 be sought to carry the pollen, and this is 

 generally found in some insect, although 

 other animals, especially snails, and birds, 

 may be the agents. It is insects, however, 

 which are the most important agents, and 

 the development of the two great divisions of plants and animals 

 Angiosperms and Insects has been very largely the result of 

 mutual adaptations. Where insects are the agents in pollination, 

 the flowers are said to be Entomophilous. 



It is extremely probable that all the primitive flowers were anemo- 

 philous, and that from these have been derived the more special- 

 ized entomophilous and ornithophilous forms. It is evidently of 

 advantage to the plant to have the great waste of pollen necessi- 

 tated by wind-pollination reduced, and this is possible when insects 

 or birds are the agents in its transfer. It is probable that ento- 

 mophily began by the casual visits of insects to flowers, attracted by 

 the pollen, which is still the principal object of visits by many 

 insects, serving as an important source of food. Flowers which had 

 more conspicuous stamens or perianth would stand a better chance 

 of visits from insects, and, from the slight variations thus started, 

 may have proceeded the development of the conspicuous flowers of 

 the modern entomophilous plants. 



Fia. <83.Plantagomajor. 

 A proterogynous plant. 

 A, younger flower, the 

 pistil mature. B, older 

 flower, with mature sta- 

 mens. 



