Shull, East, Emerson and others have more recently demonstrated this same principle and the effect produced in crossing diverse types has been related to the fact that the crosses possess a larger number of favorable growth factors.
The increased vigor and fruitfulness which almost invariably result from crossing closely related sorts of varieties is a principle of the utmost importance in our common agricultural practices, for there is a great need for more vigorous forage plants, timber and shade trees, vegetables, etc., and more prolific grains and fruits.
Self-sterile Plants.— Some plants have be come so completely modified for cross-fertiliza tion that they are sterile to their own and will not set seed unless cross-fertilized. Of our wild plants this has been found to be the case in a number of instances. Koelreuter and Gartner long ago found that some plants, namely, Verbascum phceniceum, V. nigrum and Lobelia fulgens, under certain conditions were sterile to their own pollen, but would set seed abundantly when crossed with pollen of other species. Fritz Muller proved by careful-experi ment that Eschscholtsia californica, Abutilon Darwinii and a species of begonia were sterile to their own pollen, whether fertilized with pol len from the same flower or from different flowers on the same plant, but were perfectly fertile when cross-pollinated with pollen from other plants. Darwin found that these plants in England were also largely self-sterile, but that under certain conditions the degree of fertility with their own pollen could be greatly increased. The same phenomenon of self sterility and necessity for cross-pollination is shown by some of our important cultivated plants. Mr. M. B. Waite has demonstrated that many of the varieties of pears, such as Bartlett, Anjou, etc., are largely self-sterile, producing few or no fruits when pollinated only with pollen of the same variety. In the case of varieties of pears and fruits of this kind that are propagated altogether by budding, as is well known, all of the various trees of the variety that exist are simply parts of the same in dividual, so that pollinating flowers on one Bartlett pear-tree with pollen from another Bartlett pear-tree is in effect the same as pol linating one flower on a plant with pollen from a different flower on the same plant and is what we recognize as self-fertilization. Mr. Waite and others have shown that when such sterile varieties are pollinated from a different horti cultural variety, which in reality is simply a different individual of the same species, they are rendered perfectly fertile. This discovery has proved of great practical value, as many pear orchards were planted with trees of a single variety like the Bartlett and were found for some cause to be very unfruitful.
Waite's discovery showed the true cause of this lack of fruitfulness and at the same time pointed out the natural and simple remedy of cross-fertilization. All that remained to be done in such barren orchards was to bud some of the trees over with buds of varieties which had by experiment been found to produce a good percentage of fertility in crossing on the variety concerned. Since Waite's discovery of the reason for the barrenness of certain varieties of pears many experiments have been conducted by different investigators. It is now known as a result of experiments by Waite, Waugh, Beach• and others that many varieties of plum and apple are largely sterile and require cross pollination for complete fruitfulness.
The pineapple, also, as shown by the writer's studies is an interesting example of a self sterile plant. Ordinarily the pineapple is wholly seedless, yet the flowers are so arranged that they are abundantly fertilized with their own pollen. The self-sterility is so perfect that even under these circumstances seeds set so rarely that few growers and consumers have ever seen seeds. In experiments in crossing various varieties it was found that when certain varieties were crossed seeds were formed in abundance, showing that cross-fertilization is necessary to ensure the development of perfect seeds. The explanation for the setting of such few seeds in cultivation normally is probably to be found in the fact that the proper insect pollinators are not present, and the flowers are therefore not cross-fertilized. See FERTILI ZATION IN PLANTS; FERTILIZATION IN FLOWERS.
Bibliography.—Allen, Grant, 'On the Colors of Flowers> (London 1882) ; Bailey, L. H., Breeding' (New York 1895) ; Church, A. H., 'Floral Mechanisms (Oxford 1908) ; Darwin, 'Various Contrivances by which Orchids are Fertilized by Insects' (London 1877) ; 'Effect of Cross and Self-fertilization in the Vegetable Kingdom> (London 1878) ; 'Dif ferent Forms of Flowers on Plants of the Same Species' (London 1880) ; 'The Variation of Animals and Plants under Domestication> (London 1892) ; Henslow, 'Origin of Floral Structures' (1893) ; Knuth, P. 'Handbook of Flower Pollination' (tr. by J. R. A. Davis, Oxford 1906-09); Lubbock, 'British Wild Flowers Considered in Relation to Insects> (London 1882) ; Miles, 'Stock Breeding' (1888) ; 'The Fertilization of Flowers> (London 1883) ; Swingle and Webber, 'Hybrids and Their Utilization in Plant Breeding> (Year book U. S. Department of Agriculture, Wash ington 1897) ; Waite, 'The Pollination of Pear Flowers' (Bull. 5 Div. Veg. Pathology, U. S. Department of Agriculture, Washington 1895); and Wallace, 'Darwinism> (London 1889).