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1、Mendels Paper in English Experiments in Plant Hybridization by Gregor MendelRead at the meetings of February 8th, and March 8th, 18651 Introductory RemarksExperience of artificial fertilization, such as is effected with ornamental plants in order to obtain new variations in color, has led to the exp
2、eriments which will here be discussed. The striking regularity with which the same hybrid forms always reappeared whenever fertilization took place between the same species induced further experiments to be undertaken, the object of which was to follow up the developments of the hybrids in their pro
3、geny. To this object numerous careful observers, such as Klreuter, Grtner, Herbert, Lecoq, Wichura and others, have devoted a part of their lives with inexhaustible perseverance. Grtner especially in his work Die Bastarderzeugung im Pflanzenreiche , has recorded very valuable observations; and quite
4、 recently Wichura published the results of some profound investigations into the hybrids of the Willow. That, so far, no generally applicable law governing the formation and development of hybrids has been successfully formulated can hardly be wondered at by anyone who is acquainted with the extent
5、of the task, and can appreciate the difficulties with which experiments of this class have to contend. A final decision can only be arrived at when we shall have before us the results of detailed experiments make on plants belonging to the most diverse orders. Those who survey the work done in this
6、department will arrive at the conviction that among all the numerous experiments made, not one has been carried out to such an extent and in such a way as to make it possible to determine the number of different forms under which the offspring of the hybrids appear, or to arrange these forms with ce
7、rtainty according to their separate generations, or definitely to ascertain their statistical relations. It requires indeed some courage to undertake a labor of such far-reaching extent; this appears, however, to be the only right way by which we can finally reach the solution of a question the impo
8、rtance of which cannot be overestimated in connection with the history of the evolution of organic forms. The paper now presented records the results of such a detailed experiment. This experiment was practically confined to a small plant group, and is now, after eight years pursuit, concluded in al
9、l essentials. Whether the plan upon which the separate experiments were conducted and carried out was the best suited to attain the desired end is left to the friendly decision of the reader. 2 Selection of the Experimental PlantsThe value and utility of any experiment are determined by the fitness
10、of the material to the purpose for which it is used, and thus in the case before us it cannot be immaterial what plants are subjected to experiment and in what manner such experiment is conducted. The selection of the plant group which shall serve for experiments of this kind must be made with all p
11、ossible care if it be desired to avoid from the outset every risk of questionable results. The experimental plants must necessarily: 1. Possess constant differentiating characteristics. 2. The hybrids of such plants must, during the flowering period, be protected from the influence of all foreign po
12、llen, or be easily capable of such protection.The hybrids and their offspring should suffer no marked disturbance in their fertility in the successive generations. Accidental impregnation by foreign pollen, if it occurred during the experiments and were not recognized, would lead to entirely erroneo
13、us conclusions. Reduced fertility or entire sterility of certain forms, such as occurs in the offspring of many hybrids, would render the experiments very difficult or entirely frustrate them. In order to discover the relations in which the hybrid forms stand towards each other and also towards thei
14、r progenitors it appears to be necessary that all member of the series developed in each successive generations should be, without exception, subjected to observation. At the very outset special attention was devoted to the Leguminosae on account of their peculiar floral structure. Experiments which
15、 were made with several members of this family led to the result that the genus Pisum was found to possess the necessary qualifications. Some thoroughly distinct forms of this genus possess characters which are constant, and easily and certainly recognizable, and when their hybrids are mutually cros
16、sed they yield perfectly fertile progeny. Furthermore, a disturbance through foreign pollen cannot easily occur, since the fertilizing organs are closely packed inside the keel and the anthers burst within the bud, so that the stigma bees covered with pollen even before the flower opens. This circum
17、stance is especially important. As additional advantages worth mentioning, there may be cited the easy culture of these plants in the open ground and in pots, and also their relatively short period of growth. Artificial fertilization is certainly a somewhat elaborate process, but nearly always succe
18、eds. For this purpose the bud is opened before it is perfectly developed, the keel is removed, and each stamen carefully extracted by means of forceps, after which the stigma can at once be dusted over with the foreign pollen. In all, 34 more or less distinct varieties of Peas were obtained from sev
19、eral seedsmen and subjected to a two years trial. In the case of one variety there were noticed, among a larger number of plants all alike, a few forms which were markedly different. These, however, did not vary in the following year, and agreed entirely with another variety obtained from the same s
20、eedsman; the seeds were therefore doubtless merely accidentally mixed. All the other varieties yielded perfectly constant and similar offspring; at any rate, no essential difference was observed during two trial years. For fertilization 22 of these were selected and cultivated during the whole perio
21、d of the experiments. They remained constant without any exception. Their systematic classification is difficult and uncertain. If we adopt the strictest definition of a species, according to which only those individuals belong to a species which under precisely the same circumstances display precis
22、ely similar characters, no two of these varieties could be referred to one species. According to the opinion of experts, however, the majority belong to the species Pisum sativum; while the rest are regarded and classed, some as sub-species of P. sativum, and some as independent species, such as P.
23、quadratum, P. saccharatum, and P. umbellatum. The positions, however, which may be assigned to them in a classificatory system are quite immaterial for the purposes of the experiments in question. It has so far been found to be just as impossible to draw a sharp line between the hybrids of species a
24、nd varieties as between species and varieties themselves. 3 Division and Arrangement of the ExperimentsIf two plants which differ constantly in one or several characters be crossed, numerous experiments have demonstrated that the mon characters are transmitted unchanged to the hybrids and their prog
25、eny; but each pair of differentiating characters, on the other hand, unite in the hybrid to form a new character, which in the progeny of the hybrid is usually variable. The object of the experiment was to observe these variations in the case of each pair of differentiating characters, and to deduce
26、 the law according to which they appear in successive generations. The experiment resolves itself therefore into just as many separate experiments are there are constantly differentiating characters presented in the experimental plants. The various forms of Peas selected for crossing showed differen
27、ces in length and color of the stem; in the size and form of the leaves; in the position, color, size of the flowers; in the length of the flower stalk; in the color, form, and size of the pods; in the form and size of the seeds; and in the color of the seed-coats and of the albumen cotyledons. Some
28、 of the characters noted do not permit of a sharp and certain separation, since the difference is of a more or less nature, which is often difficult to define. Such characters could not be utilized for the separate experiments; these could only be applied to characters which stand out clearly and de
29、finitely in the plants. Lastly, the result must show whether they, in their entirety, observe a regular behavior in their hybrid unions, and whether from these facts any conclusion can be reached regarding those characters which possess a subordinate significance in the type. The characters which we
30、re selected for experiment relate: 1. To the difference in the form of the ripe seeds. These are either round or roundish, the depressions, if any, occur on the surface, being always only shallow; or they are irregularly angular and deeply wrinkled . 2. To the difference in the color of the seed alb
31、umen . The albumen of the ripe seeds is either pale yellow, bright yellow and orange colored, or it possesses a more or less intense green tint. This difference of color is easily seen in the seeds as their coats are transparent. 3. To the difference in the color of the seed-coat. This is either whi
32、te, with which character white flowers are constantly correlated; or it is gray, gray-brown, leather-brown, with or without violet spotting, in which case the color of the standards is violet, that of the wings purple, and the stem in the axils of the leaves is of a reddish tint. The gray seed-coats
33、 bee dark brown in boiling water. 4. To the difference in the form of the ripe pods. These are either simply inflated, not contracted in places; or they are deeply constricted between the seeds and more or less wrinkled . 5. To the difference in the color of the unripe pods. They are either light to
34、 dark green, or vividly yellow, in which coloring the stalks, leaf-veins, and calyx participate.* 6. To the difference in the position of the flowers. They are either axial, that is, distributed along the main stem; or they are terminal, that is, bunched at the top of the stem and arranged almost in
35、 a false umbel; in this case the upper part of the stem is more or less widened in section . 7. To the difference in the length of the stem. The length of the stem is very various in some forms; it is, however, a constant character for each, in so far that healthy plants, grown in the same soil, are
36、 only subject to unimportant variations in this character. In experiments with this character, in order to be able to discriminate with certainty, the long axis of 6 to 7 ft. was always crossed with the short one of 3/4 ft. to 1 and 1/2 ft. Each two of the differentiating characters enumerated above
37、 were united by cross-fertilization. There were made for the 1st trial60 fertilizations on 15 plants.2nd trial58 fertilizations on 10 plants.3rd trial35 fertilizations on 10 plants.4th trial40 fertilizations on 10 plants.5th trial23 fertilizations on 5 plants.6th trial34 fertilizations on 10 plants.
38、7th trial37 fertilizations on 10 plants.*One species possesses a beautifully brownish-red colored pod, which when ripening turns to violet and blue. Trials with this character were only begun last year.From a larger number of plants of the same variety only the most vigorous were chosen for fertiliz
39、ation. Weakly plants always afford uncertain results, because even in the first generation of hybrids, and still more so in the subsequent ones, many of the offspring either entirely fail to flower or only form a few and inferior seeds. Furthermore, in all the experiments reciprocal crossings were e
40、ffected in such a way that each of the two varieties which in one set of fertilizations served as seed-bearer in the other set was used as the pollen plant. The plants were grown in garden beds, a few also in pots, and were maintained in their natural upright position by means of sticks, branches of
41、 trees, and strings stretched between. For each experiment a number of pot plants were placed during the blooming period in a greenhouse, to serve as control plants for the main experiment in the open as regards possible disturbance by insects. Among the insects which visit Peas the beetle Bruchus p
42、isi might be detrimental to the experiments should it appear in numbers. The female of this species is known to lay the eggs in the flower, and in so doing opens the keel; upon the tarsi of one specimen, which was caught in a flower, some pollen grains could clearly be seen under a lens. Mention mus
43、t also be made of a circumstance which possibly might lead to the introduction of foreign pollen. It occurs, for instance, in some rare cases that certain parts of an otherwise normally developed flower wither, resulting in a partial exposure of the fertilizing organs. A defective development of the
44、 keel has also been observed, owing to which the stigma and anthers remained partially covered. It also sometimes happens that the pollen does not reach full perfection. In this event there occurs a gradual lengthening of the pistil during the blooming period, until the stigmatic tip protrudes at th
45、e point of the keel. This remarkable appearance has also been observed in hybrids of Phaseolus and Lathyrus. The risk of false impregnation by foreign pollen is, however, a very slight one with Pisum, and is quite incapable of disturbing the general result. Among more than 10,000 plants which were c
46、arefully examined there were only a very few cases where an indubitable false impregnation had occurred. Since in the greenhouse such a case was never remarked, it may well be supposed that Bruchus pisi, and possibly also the described abnormalities in the floral structure, were to blame. 4 The Form
47、s of the HybridsExperiments which in previous years were made with ornamental plants have already affording evidence that the hybrids, as a rule, are not exactly intermediate between the parental species. With some of the more striking characters, those, for instance, which relate to the form and si
48、ze of the leaves, the pubescence of the several parts, etc., the intermediate, indeed, is nearly always to be seen; in other cases, however, one of the two parental characters is so preponderant that it is difficult, or quite impossible, to detect the other in the hybrid. This is precisely the case
49、with the Pea hybrids. In the case of each of the 7 crosses the hybrid-character resembles that of one of the parental forms so closely that the other either escapes observation pletely or cannot be detected with certainty. This circumstance is of great importance in the determination and classification of the forms under which the offspring of the hybrids appear. Henceforth in this paper those character
