Plant breeding

“ The rapid development of the science and art of breeding places us to-day in position to secure improvements much more rapidly than has been done in the past. It would not be astonishing if from 25 to 50 years of careful, intelligent breeding would accomplish with a wild plant what has required many centuries under the crude methods of our ancestors.
It may be asked why we should be in haste to take up the improvement of our native plants. ”

“ In other cases it is only possible to pick out the individual plants which breed true by growing their seed and observing how it behaves. If it produces progeny all of which are like the plant from which the seed was obtained, that plant is a fixed type and will breed true continuously in the future. The final result of the experiment was to obtain in three years from the time the cross was made, four fixed types which subsequent experience has shown breed true continuously, a long eared bearded type, a short eared beardless type, a long eared beardless type and a short eared bearded type. ”

“ When a gardener wishes to obtain variations of a plant from which to select and establish a new breed, he deliberately sets to work to disturb—to shake up, to act upon in a tentative, experimental way—the germ-plasm of one or more parent plants by change of soil, climate, food and often by cross-fertilizing them with another breed or variety. ”

“ The balance of the reddish-purple plants, which comprise about two-thirds of the second generation, will, if their seeds are germinated, produce colored as against white-flowered progeny in the three-to-one ratio. In other words, these artificial crosses, made by the plant breeder, and this splitting up of hybrids which has been many times verified, are seen to be very fertile causes of the origin of new forms of plant life, if only the factor and character differences in the ancestry be sufficiently complex. ”

“ But suppose we are dealing with red clover where the flowers are small, almost sterile with their own pollen and produce only one seed. In this crop, the long and tedious method of continuous selection just mentioned must be used, for there is no other way. This method is often called the pedigree-culture method. The main idea of the plan is that the seeds of single plants are grown in isolated plots, and the character of the mother plant judged by the characters of the progeny. ”

“ Indeed, the faculty of spontaneous reproduction and perpetuation necessarily supposes a greater power of accommodation, within a certain range, than we find in most domesticated plants, for it would rarely happen that the seed of a wild plant would fall into ground as nearly similar, in composition and condition, to that where its parent grew, as the soils of different fields artificially prepared for growing a particular vegetable are to each other. ”

“ We must not overlook the fact, so well established, that one of the greatest points to be gained by plant migration is to enable different stocks of a species to be cross fertilized, and thereby improved in vigor and productiveness. ”

“ Equally efficient in the hands of the cultivator has been another method of producing new forms—namely, hybridization. If the pollen of a plant be transferred to the stigma of a related species, offspring is often produced; and the product is a batch of plants intermediate in characters between the two parents, and generally uniform in appearance. ”

“ There must, as in the wind-carried pollen, be a tremendous wastage, yet sufficient quantities of it do fertilize the females, particularly in the ditch grass, which fruits very freely.
Whether it be any of the various contrivances for insect fertilization, or by the winds, or, as in the eelgrass, by the water, the climax of the flower’s life is always reached in this act. For all annuals the plants, also, begin to die down then, a process that is completed with the production of seed, which is, of course, the object of all those varied modes of fertilization. ”

“ By fertilizing white flowers with pollen from red ones we can almost always get seeds that will develop into plants bearing flowers that are not white. What is true of color is true of other characteristics of the plant, such, for instance, as size and shape of leaves, habit of growth, size, shape, and quality of fruit, etc. Thus by careful cross-fertilization, we are able to produce not only beautiful and new blossoms, but also many delicious new fruits. ”

“ How many animals there are which will not breed, though kept in an almost free state in their native country! This is generally, but erroneously attributed to vitiated instincts. Many cultivated plants display the utmost vigour, and yet rarely or never seed! In some few cases it has been discovered that a very trifling change, such as a little more or less water at some particular period of growth, will determine whether or not a plant will produce seeds. ”

“ If, on flying away, he next visits another male blossom on the same plant, he only collects still more pollen. But if he happens to flit off to a female flower, he brushes off some of the pollen, as he passes, on to the feathery, sensitive surface protruded by the plant right in his path, on purpose to meet him. In this way, each female blossom makes perfectly certain of due fertilization from a separate organism; and such cross-fertilization, as Darwin has shown, produces in the long run the most fertile seeds, and the strongest, heartiest, and most vigorous seedlings. ”

“ Here in most cases the pollination is done by hand.
We noticed that nature provides that most of the flowers shall be cross pollinated. This is particularly true of the flowers of the fruit trees, and for this reason it is impossible to get true varieties of fruit from seed. For example, if we plant seeds of the wine sap apple, the new trees produced from them will not produce the same kind of apple but each tree will produce something different and they will very likely all be poorer than the parent fruit. ”

“ Just as that small portion of nourishing matter which is deposited in the seed feeds the embryo when it germinates, so the much larger portion deposited in the tuber nourishes its buds, or eyes, when they likewise grow, the next spring, into new plants. And the great supply enables them to shoot with a greater vigor at the beginning, and to produce a greater amount of vegetation than the seedling plant could do in the same space of time ”

“ I have referred to American fruit-trees, because we are not in this case troubled with doubts whether or not the varieties are seedlings which have escaped from cultivation. Transplanting any superior variety, or sowing its seeds, hardly implies more forethought than might be expected at an early and rude period of civilisation. Even the Australian barbarians "have a law that no plant bearing seeds is to be dug up after it has flowered;" ”

“ The reason for this difference in reaction between native and culture formations seems evident. In harvesting, not merely the grain, but the stems and leaves, and in gardening often the root also, are removed, so that the plant makes little or no return to the soil. In nature, annual plants return to the ground every year all the solid matter of roots, stems, leaves, and fruits, with the exception of the relatively small number of seeds that germinate. ”

“ The reproduction of the higher plants takes place either asexually by the formation of buds or organs answering thereto, or sexually by the production of an embryo plant within the seed. The conditions requisite for the growth, development and reproduction of plants are, in general terms, exposure, at the proper time, to suitable amounts of light, heat and moisture, and a due supply of appropriate food. ”

“ We should bear in mind that pollen must be carried by the bees from flower to flower on the same large branching stem much more abundantly than from plant to plant; and in the case of plants the flowers of which are in some degree dichogamous, those on the same stem would be of different ages, and would thus be as ready for mutual fertilisation as the flowers on distinct plants, were it not for the prepotency of pollen from another variety. ”

“ The possibilities of the pedigree-culture are by no means exhausted with covering the above points, especially if the plants chosen for the objects happen to be capable of vegetative reproduction, that is by cuttings or slips, in which case comparisons should be made of progenies grown from seeds of the plant with wide leaves with individuals grown, from slips of the same, and also compared with others from the extreme end of the series. ”

“ As a fertilising rain quickly calls into existence the most varied assortment of plants on a barren steppe, so activity now reigns in a field which men formerly left deserted. This development of the biology of flowers is of importance not only on theoretical grounds but also from a practical point of view. The rational breeding of plants is possible only if the flower-biology of the plants in question (i.e. the question of the possibility of self-pollination, self-sterility, etc.) ”

“ If species and varieties can be distinguished only by factor differences, as the plant breeders no doubt correctly insist, it becomes obvious enough that we have in cross-fertilization to consider not alone the factor differences of the pistillate or female, but also of the staminate or male contribution to the union, and how these are reflected in the progeny. Our knowledge of this has practically all been based on work done on cultivated plants under control conditions and it shows some interesting developments which occur from crossing. ”

“ Propagation and Cultivation.—It is propagated from seeds, which should be sown in spring, as soon as the ground has become well settled. They may be sown where the plants are to remain; or in a nursery-bed, to be afterwards transplanted. The plants should be grown in rows eighteen inches or two feet apart, and about a foot apart in the rows. The after-culture consists simply in keeping the ground loose, and free from weeds. The plant is said to yield a much greater amount of herbage if the top is broken or cut off when it is about half grown. ”

“ If the plant fertilizes itself without the aid of insects, as do some evening-primroses, the seed saved from the native locality may prove wholly pure, and if it does give rise to a uniform progeny the constancy of the race may be assumed to be proved, provided that repeated trials do not bring to light any exceptions. If the offspring shows more than one type, cross-fertilization is always to be looked to as the most probable cause, and should be excluded, in order to sow pure seeds. ”

“ Reproduction and the ultimate scattering of fruits and seeds are absolutely necessary in order that colonies of plants may reach new localities. It is evident that plants best fitted to scatter their seeds, or place fruits containing the seeds some little distance from the parent plants, are the ones which will spread most rapidly. A plant, if it is to advance into new territory, must get its seeds there first. Plants which are best fitted to do this are the most widely distributed on the earth. ”

“ By Seeds.—This may be called the natural means of increasing the number of any particular kind of plant, but it is to be remembered that we do not by that means secure an exact reproduction of the parent, especially in the case of plants raised or evolved in the course of generations by hybridization and selection. We may get a progeny very closely resembling it, yet each plant possessing a distinct individuality of its own; or we may get a progeny very unlike the parent, or a mixed progeny showing various degrees of divergence. Many seeds will grow freely if sown in a partially ripened state ”

“ Although the plants raised during ten successive generations from crosses between distinct yet inter-related plants almost invariably exceeded in height, constitutional vigour, and fertility their self-fertilised opponents, it has been proved that seedlings raised by intercrossing flowers on the same plant are by no means superior, on the contrary are somewhat inferior in height and weight, to seedlings raised from flowers fertilised with their own pollen. ”

“ A study of this species, as well as of plant-life in general, teaches us that nature abhors self-fertilization and, as a rule, so develops plants that two individuals of the same species are essential to the production of seed.
This species especially enjoys nature, and is not easily cultivated. A few florists have succeeded in producing mature plants with fair results, but it may be stated that even transplanting, with much soil attached to the roots, to a soil identical with the native, results in a weakened development. ”

“ Here the rule prevails that every plant must be fertilized with pollen of its own, or with pollen of other individuals of known and recorded origin. The visits of insects must be guarded against, and no seeds should be saved from flowers which have been allowed to open without this precaution. Then the seeds of each individual must be saved and sown separately, so as to admit of an appreciation, and if necessary, a numerical determination of the nature of its progeny. And last but not least the experiments should be conducted in a similar manner during a series of successive years. ”

“ Heterogone Trimorphism, in which there is a mid-length as well as a long and a short set of stamens and style; occurs in Lythrum Salicaria and some species of Oxalis.
342. There must be some essential advantage in cross fertilization or cross breeding. Otherwise all these various, elaborate, and exquisitely adjusted adaptations would be aimless. Doubtless the advantage is the same as that which is realized in all the higher animals by the distinction of sexes.
§ 2. ACTION OF POLLEN, AND FORMATION OF THE EMBRYO.
343. Pollen-growth. ”

“ Doubleness is strongly inherited. No one has produced, as Lindley remarks, [97] double flowers by promoting the perfect health of the plant. On the contrary, unnatural conditions of life favour their production. There is some reason to believe that seeds kept during many years, and seeds believed to be imperfectly fertilised, yield double flowers more freely than fresh and perfectly fertilised seed. [98] Long-continued cultivation in rich soil seems to be the commonest exciting cause. ”