(3) The stamens and pistils may be so placed with reference to each other that pollination can be brought about only by outside assistance.
Artificial Cross-pollination and its Practical Benefits to Man.—Artificial cross-pollination is practiced by plant breeders and can easily be tried in the laboratory or at home. First the anthers must be carefully removed from the bud of the flower so as to eliminate all possibility of self-pollination. The flower must then be covered so as to prevent access of pollen from without; when the ovary is sufficiently developed, pollen from another flower, having the characters desired, is placed on the stigma and the flower again covered to prevent any other pollen reaching the flower. The seeds from this flower when planted may give rise to plants with the best characters of each of the plants which contributed to the making of the seeds.
[1] For an excellent account of cross-pollination of this flower, the reader is referred to W. C. Stevens, Introduction to Botany. Orchids are well known to botanists as showing some very wonderful adaptations. A classic easily read is Darwin, On the Fertilization of Orchids.
Reference Books
elementary
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Andrews, A Practical Course in Botany, pages 214-249. American Book Company.
Atkinson, First Studies of Plant Life, Chaps. XXV-XXVI. Ginn and Company.
Coulter, Plant Life and Plant Uses, pages 301-322. American Book Company.
Dana, Plants and their Children, pages 187-255. American Book Company.