REPRODUCTION AND DISPERSAL 869 



significant of all is the prevalence of vegetative reproduction in nearly 

 all the higher plants. There is not a single case known in which the 

 indefinite continuance of vegetative reproduction causes the slightest 

 deterioration. Indeed, there are many plants in which crossing has 

 essentially disappeared (as the duckweeds and the horn worts). Nor 

 is there any evidence of deterioration in the species that are persistently 

 parthenogenetic. It is to be noted that the plants which in experiments 

 have shown benefit from crossing are chiefly garden varieties that have 

 been much hybridized. No benefit from crossing has been shown in 

 natural species that have long been pure. Nor is there good evidence 

 that crossing promotes variability. Among the most variable of plants 

 are the cleistogamous violets and such parthenogenetic genera as Hiera- 

 cinm and Taraxacum. 



The possible disadvantages of close pollination. — Even if there are no 

 conspicuous advantages in cross pollination, there may be disadvantages 

 in close pollination. Such a view seems particularly plausible in the 

 case of those orchids in which own pollen seems to be prejudicial and 

 possibly even poisonous to the stigma (or the stigma to the germinating 

 own pollen), perhaps in a way analogous to the excreta of root hairs, 

 except that here the deleterious effects concern only individuals of a 

 species, and not the species as a whole. If this conception is valid, it 

 may account for the occasional impotence of own pollen; in species 

 where foreign pollen is prepotent, the deleterious influence of own pollen 

 may be considered to be less marked. In the large number of species 

 with potent own pollen it may be supposed that such deleterious effects 

 are wanting. The benefits of cross pollination and the disadvantages 

 of close pollination have been too much emphasized. Close pollination 

 and its essential equivalent, geitonogamy, are extremely common in 

 nature, nor must it be forgotten, also, that many of the important plant 

 and animal races utilized by man have reached their present state of 

 commercial perfection by the most careful inbreeding. 



The protective features of flowers. — The calyx. — Flowers are among 

 the most delicate of plant organs, the ephemeral petals and the stamens 

 and pistils with their gametophytes and embryos being particularly 

 sensitive. Foremost among the protective organs is the calyx (fig. 1136), 

 which, during development, often is the only exposed floral organ, and 

 commonly is much less delicate than are the structures it encloses. 

 Among the dangers to which the developing corolla and the essential 

 organs might otherwise be exposed are those arising from rain, drought, 



