The matter of fertilization underlies the whole subject of seed production, for on the fertilization of the seed depends the purity as well as the vigor of a variety. A flower is cross fertilized when its ovules are impregnated by pollen of another flower; self-fertilized when they are impregnated by pollen from its own stamens.

The adaptations for cross fertilization are to numerous to be described in detail. Two great external agencies are concerned in the work, the wind and insects. the colors, odors, and irregular shapes of flowers and the secretion of honey are correlated with cross fertilization by insects. Insects visit flowers for the sweets they find, and are undoubtedly attracted by color and odor. While collecting the honey, insects are dusted with pollen, which, passing to other flowers, they deposit on their stigmas. Many flowers are so arranged that only bees and insects large enough to pollinate the flower can obtain the honey. This end is secured by the irregularity of the flower and in various other ways. Either the lips of the flower are so firmly closed that only a large insect can force them apart, or the throat is filled with hairs which effectually exclude unwelcome guests, or the honey is at the bottom of a long tube to which only the proboscis of a large moth or bee can reach.

The arrangements by which cross pollination is secured are principally of three kinds. (1) There may be some peculiarity in the structure of the flower that favors cross pollination: and almost or quite prevents self pollination; (2) the sexes may be in different flowers; (3) the anthers and pistils of the same flower may mature at different times.

    The peculiarities of structure are numerous and varied. A common type is found in the flower of the pea family; for instance, in that of red clover (Trifolium pratense). The flowers are visited by the bumblebee, whose long proboscis can reach down into the tube, at the bottom of which the honey is secreted. Smaller bees can not secure the honey, but they collect pollen and doubtless aid in fertilization while so doing. The stigma stands out above the anthers, and a bee, thrusting his head into a flower, would first brush against the stigma, leaving some pollen from a flower previously visited, and then dust itself afresh with pollen, to be carried in turn to the next flower. Some farm operations depend upon these insect visits. Where mammoth clover is grown for seed, it is pastured or clipped in the early part of the season. This is done that the plants may not bloom before "bee time," for if they did there would be no seed. Bumblebees had to be imported into Australia before red clover seed could be raised there. It is said that when insects are excluded not one-tenth of the flowers are fertile.

In the cabbage family arrangements are such that self-fertilization can take place if cross fertilization fails. In cabbage (Brassica oleracea) the honey is secreted at the bottom of the corolla tube. An insect sucking the honey would touch the stigma and the anther of one of the short stamens. At the next flower the pollen thus collected would most likely be deposited on the stigma. In case cross fertilization fails, the long stamens bend over and pollinate the stigma. That cross fertilization frequently occurs is proved, however, by the difficulty of keeping the varieties of cabbage, turnips, or other cruciferous vegetables from mixing.

The case in which cross fertilization is insured by having the sexes in different flowers is represented among our garden vegetables by the cucurbits. In pumpkins, squashes, cucumbers, and melons the male flowers appear first, followed by the female. Here cross fertilization is inevitable, and mixing invariably occurs when several varieties of a species are grown near to one another.

The wind fertilized flowers are represented among our common economic plants by the grasses and Indian corn. In these the flowers are simple, without odor, nectar, or conspicuous color, thus presenting no attraction to insects. The anthers are born on long, delicate filaments, which enable them to shake out their light and dry pollen with every breath of wind. Everyone has noticed how the pollen falls in showers from the tassels of Indian corn.

    Flowers in which the anthers mature before the pistil are common among our ornamental plants, the gentians, campanulas, and Clorodendron being conspicuous examples. In the plantains, which are among our greatest weed pests, the pistils mature before the anthers. In these the order of blossoming is from the base of the spike up, and in young spikes the stages of flowering can be traced on one spike; the younger flowers at the apex showing only pistils, the middle flowers old pistils and young anthers, the lower ones withered pistils and ripe anthers.

Cross fertilization may take place between flowers on the same plant or between flowers on different plants. In the s latter case two distinct individuals enter into the union. They bring to this union those differences of constitution and habit which always exist between individuals, emphasized, perhaps by differences in the conditions under which their ancestors have lived for one or more generations. In self-fertilization, on the other hand, there is the closest possible inbreeding. The conditions under which the sexual organs have matured are the same; they will therefore differ but little in their constitution. It would seem reasonable that seed produced by crossing different plants should give rise to progeny more vigorous and productive than that resulting from self fertilization.

Darwin proved by a long series of experiments that cross fertilization is beneficial and that continued self-fertilization is injurious to the species. He crystallized his conclusions in the famous dictum, "Nature abhors perpetual self-fertilization."

In the course of his experiments, Darwin used many species from widely different orders, and in most cases several experiments were conducted with the same species. Many of the plants were grown for more than one generation from seeds produced by hand pollinated flowers; in one case, that of the morning glory (Japonica purpurea) for ten generations. Darwin found that as a rule, the plants raised from seeds produced by cross fertilization exceeded in height, weight, and fertility those raised from seeds produced by self fertilization. The same results were obtained by Bailey in growing eggplants on a large scale. The cross-bred plants "were characterized throughout the season by great sturdiness and vigor of growth. They grew more erect and taller than other plants near by grown from commercial seed. They were the finest plants I had ever seen." The following summer 2,500 plants were grown from seed taken from this patch and of these he says, "Again the plants were remarkably robust and healthy, with fine foliage, and they grew erect and tall - an indication of vigor."

The degree of the relationship of the plants used in the cross has much to do with the benefit derived. Crosses between plants grown for generations under the same conditions tend to approximate the results of self-fertilization. But the introduction of fresh-stock - that is, of plants grown under other conditions of soil, moisture, climate, or care - puts new vigor into the cross. In Darwin's experiments plants of morning glory that had been intercrossed for nine generations were crossed with fresh stock and compared with plants intercrossed for ten generations. They exceeded the latter in height as 100 exceeds 78. Cabbages were compared by weight, and the plants resulting from a cross with fresh stock upon the second intercrossed generation were to the third intercrossed generation as 100 to 22. More examples might be given, but these are enough to show the immense advantage of introducing fresh stock.

As has already been said, the purity as well as the vigor of a variety depends upon the fertilization of the seed. While cross fertilization has been clearly shown to be productive of more vigorous plants and therefore a benefit so far as the life of the species is concerned, there are other matters of vital importance to the farmer. The first of these is the purity of the variety. The seed must be genuine, that is it must reproduce the variety from which it purports to come. No matter how well the seed germinates nor how vigorous the plants, if they are not of the variety wanted the crop is at best a partial failure. While crossing between plants of the same variety is beneficial, the more so if the ancestors were not grown under the same conditions, crossing between varieties of a species should, as a rule, be guarded against. Intelligent crossing of varieties, or of species even, may lead to good results, but indiscriminate crossing can only result in the loss of well-established types.

The varieties of most of our common garden plants cross readily. The great pea and bean growers are careful not to grow two varieties of either near each other. Some insist upon but one variety being grown on the same farm, while others permit more than one on a farm, but specify the distance they shall stand apart, from 10 to 40 rods [165 to 660 feet] being usually required.

In the cabbage family, to which many of our vegetables belong, cross fertilization is not uncommon. The varieties of cabbage, kale, cauliflower, brussels sprouts, and kohlrabi all belong to one species (Brassica oleracea) and cross freely. To raise any variety for seed, it must be grown in large patches, away from any variety with which it will cross. Where only a few plants produce seed, there is every chance that bees may bring pollen of another variety and the plants raised from that seed not be true to name. The varieties of corn cross readily and can not be grown near each other without danger of mixing. This may occur even when the varieties are considerable distances apart. We learn from a seedsman of high standing that in his experience corn in one field has become mixed with that in another 6 miles distant. In his opinion crows that fed on both fields carried the pollen. Frequently the effects of the crosses are visible the same season in colored grains. When this occurs, the mixing is easily detected and the corn can be discarded, but often there is no such indication, and a mongrel variety the following summer is the first indication the farmer has that his seed corn was not genuine.

Pumpkins, squashes, melons, and cucumbers are fertilized by insects, and when different varieties of the same species are grown near each other pure seed is out of the question. Even when the varieties are grown some distance apart there is still danger of mixing. A seedsman states that he has known watermelons to cross when grown a mile apart. Bees carry pollen long distances, and when flying from one field to another are likely to leave foreign pollen on the stigmas of the flowers they visit. Experience has shown that tomatoes will cross in the field. Six varieties were grown on the Cornell Experimental Grounds, and from the seed saved eighty-seven plants were grown, six of which were evidently crosses. It is evident, then that garden seeds grown on a small scale are extremely likely to mix when two or more varieties of the same plant are grown together.