A Civic Biology, Presented in Problems. George W. Hunter
pollen from an anther, deposits an egg in the ovary of the pistil, and then rubs its load of pollen over the stigma of the flower. The young hatch out and feed on the young seeds which have grown because of the pollen placed on the stigma by the mother. The baby caterpillars eat some of the developing seeds and later bore out of the seed pod and escape to the ground, leaving the plant to develop the remaining seeds without further molestation.
The pronuba pollinating the pistil of the yucca.
Pod of yucca showing where the young pronubas escaped.
The fig insect (Blastophaga grossorum) is another member of the insect tribe that is of considerable economic importance. It is only in recent years that the fruit growers of California have discovered that the fertilization of the female flowers is brought about by a gallfly which bores into the young fruit. By importing the gallflies it has been possible to grow figs where for many years it was believed that the climate prevented figs from ripening.
Other Flower Visitors.—Other insects besides those already mentioned are pollen carriers for flowers. Among the most useful are moths and butterflies. Projecting from each side of the head of a butterfly is a fluffy structure, the palp. This collects and carries a large amount of pollen, which is deposited upon the stigmas of other flowers when the butterfly pushes its head down into the flower tube after nectar. The scales and hairs on the wings, legs, and body also carry pollen.
A humming bird about to
cross-pollinate a lily.
Flies and some other insects are agents in cross-pollination. Humming birds are also active agents in some flowers. Snails are said in rare instances to carry pollen. Man and the domesticated animals undoubtedly frequently pollinate flowers by brushing past them through the fields.
Pollination by the Wind.—Not all flowers are dependent upon insects or other animals for cross-pollination. Many of the earliest of spring flowers appear almost before the insects do. Such flowers are dependent upon the wind for carrying pollen from the stamens of one flower to the pistil of another. Most of our common trees, oak, poplar, maple, and others, are cross-pollinated almost entirely by the wind.
A cornfield showing staminate and pistillate flowers, the latter having become grains of corn. An ear of corn is a bunch of ripened fruits.
Flowers pollinated by the wind are generally inconspicuous and often lack a corolla. The anthers are exposed to the wind and provided with much pollen, while the surface of the stigma may be long and feathery. Such flowers may also lack odor, nectar, and bright color. Can you tell why?
Imperfect Flowers.—Some flowers, the wind-pollinated ones in particular, are imperfect; that is, they lack either stamens or pistils. Again, in some cases, imperfect flowers having stamens only are alone found on one plant, while those flowers having pistils only are found on another plant of the same kind. In such flowers, cross-pollination must of necessity follow. Many of our common trees are examples.
Other Cases.—The stamens and pistil ripen at different times in some flowers. The "Lady Washington" geranium, a common house plant, shows this condition. Here also cross-pollination must take place if seeds are to be formed.
The flower of "Lady Washington" geranium, in which stamens and pistil ripen at different times, thus insuring cross-pollination. A, flower with ripe stamens; B, flower with stamens withered and ripe pistil.
Summary.—If we now collect our observations upon flowers with a view to making a summary of the different devices flowers have assumed to prevent self-pollination and to secure cross-pollination, we find that they are as follows:—
(1) The stamens and pistils may be found in separate flowers, either on the same or on different plants.
(2) The stamens may produce pollen before the pistil is ready to receive it, or vice versa.
(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.
Lubbock, Flowers, Fruits, and Leaves, Part I. The Macmillan Company.
Needham, General Biology, pages 1–50. The Comstalk Publishing Company.
Newell, A Reader in Botany, Part II, pages 1–96. Ginn and Company.
Sharpe, A Laboratory Manual in Biology, pages 43–48. American Book Company.
advanced
Bailey, Plant Breeding. The Macmillan Company.
Campbell, Lectures on the Evolution of Plants. The Macmillan Company.
Coulter, Barnes, and Cowles, A Textbook of Botany, Part II. American Book Company.
Darwin, Different Forms of Flowers on Plants of the Same Species, D. Appleton and Company.
Darwin, Fertilization in the Vegetable Kingdom, Chaps. I and II. D. Appleton and Company.
Darwin, Orchids Fertilized by Insects, D. Appleton and Company.
Lubbock, British Wild Flowers. The Macmillan Company.
Müller, The Fertilization of Flowers. The Macmillan Company.
IV. THE FUNCTIONS AND COMPOSITION OF LIVING THINGS
Problems.—To discover the functions of living matter.
(a) In a living plant.
(b) In a living animal.