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Cross-Fertilization in Plants

pollen, plant, flowers, corn, wind-fertilized, field, wind and sexuality

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Methods by Which Cross-Fertilization is a knowledge of the sex distinction of animals dates from the dawn of human history, the sexuality of plants remained unknown until about two centuries ago. Experi mental proof of the sexuality of plants was published for the first time by Camerarius in 1691, and only after this discovery was the function of pollen known and its necessity in seed formation recognized. The first recorded hybrid was made by Thomas Fairchild about 1711, and very careful studies of plant hybrids were published by Koelreuter in 1760. With all this accumulating knowledge of the sexuality and crossing of plants, it is surprising that the true meaning and significance of the flower and its various adaptations to secure cross-fertiliza tion was not perceived until Christian Conrad Sprengel completed his researches and in 1793 published his now classical work entitled, The Secret of Nature Discovered in the Form and Fertilization of Flowers.) Sprengel discovered the principal facts connected with the cross pollination of flowers by insects. He recognizes the true significance of honey and of bright colored flowers, that they were hut means to attract insects to the flowers and that the in sects carried pollen from one plant to another and aided in securing cross-fertilization. He, however, failed to recognize that the plant derived any benefit from the cross-fertilization. It was left for Charles Darwin, the great English naturalist, to point out that certain species of flowers are entirely dependent for fertilization on the transfer of the pollen from one plant to another and that self-fertilization is in the majority of cases actually injurious, resulting in loss of vigor in the progeny. The transfer of pollen in cross-fertilized plants is generally accomplished through the agency of the wind, water, insects or birds, and the various devices that have been adopted by the plant to secure crossing form an interesting and inexhaustible field for study and observa tion.

Wind-fertilized or anemophilous, flowers are those so modified as to depend upon the wind to secure cross-fertilization by carrying the pollen of one plant to the stigmas of another. Anemophilous plants are characterized by having dry and powdery pollen, which is very abun dant and light_ and easily carried by the wind. In most cases also the pistils are large and feathery, with large, sticky stigmas, presenting an abundant surface to catch the pollen floating in the air. In wind-fertilized plants there is a

great opportunity for loss of pollen and it thus becomes necessary that an abundant supply should be formed. The various pine-trees, of which there are large forests in some parts of America, are wind-fertilized and form enor mous masses of pollen. In this case, the pollen grains are provided with two lateral wing-like extensions which are supposed to be of service in making the pollen lighter and easier to blow about Several instances are recorded where the decks of vessels at sea have been covered by a rain of pollen which, in some cases, must have been carried a distance of some 400 miles. Corn, or maize, forms a familiar example of a wind-fertilized plant. The pollen is produced in great abundance in the stamens of the tassel, which forms the upper part of the stalk. When the pollen is mature the stamens protrude from the flower and the slightest jarring of the plant by the wind causes the pollen to fall in a cloud, and as the plants are grown near to gether, some' grains are almost certain to lodge on the pistils, or silks, of the ear of another plant. The silks are long and are covered with numerous stigmatic hairs so that the opportunity for plants to catch floating pollen and be cross-fertilized, in an ordinary field where numerous plants are grown, is very great. Experiments prove that cross-pollination is so universal that it is very difficult to keep varieties of corn pure. If different varieties are grown near each other cross-fertilization is cer tain to occur and impure seed results. Vilmorin found by careful experiments that plants of different varieties have to be separated by at least 1,000 feet to prevent cross-fertilization, and this distance is by no means sufficient if strong winds blow over one field of corn in such a direction as to carry the pollen from it toward another field of corn. Nevertheless it is well known that a number of plants must be planted near together to ensure thorough pollination. Plants standing alone at a distance from other corn plants seldom produce well-filled ears. The flowers of wind-fertilized plants are usually green, or greenish, inconspicuous, and have no odor or nectar. The flowers are ordinarily regular in form and they frequently appear be fore the leaves, though this is by no means universal.

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