The writer starts from the behaviour, which in rinciple is quite the same, of other characters, which submitted to the analysis of crossing have revealed themselves undoubtedly polygenous, that is determined by several Mendelian factors.

66

The most important are the factors which operate in the same direction (gleichsinnige Faktoren); a character, e. g. the red colour in wheat, may be built up by several factors, each of which has the same exterior effect, that is, each of them possesses the power of determining the appearance of the character. (For this phenomenon Lang proposed the name polymery" and Plate that of "homomery "). The black colour of the glumes of oats may be determined by two factors, instead of by only one as is generally the case. Then in F2 the segregation 3 black: I white according to the simple Mendelian scheme does not take place, but instead the segregation 15 black: I white. There are two black factors, A and B, each of which is capable of producing the black colour. After crossing with a white variety (AB ab), a black Fr plant is formed, which produces the four kinds of gametes AB, ab, aB and ab; from the 16 combinations of these gametes only one (ab ab) will be white, because all the combinations into which either one or both of the factors A and B enter will be black. The ratio of 15 black: I white in F2 is thus produced, as may be seen from the following figure:

Of the 15 black F2 plants 7 (1-7) remain constantly black in F3; 4 (8-11) segregate again in the ratio 15 black: I white, and 4 (12-15) in the ratio 3 black: I white.

Segregation follows the same law when three factors operating in the same direction are present, as in the red colour of the grain in wheat. Crossing of a three-factor red line, ABC, with a white one, abc, produces in the Fr plant eight kinds of gametes ABC, ABC, AbC, aBC, Abc, aBc, abC, and abc. Of the 67 possible combinations 63 are red and only one (abc X abc) is white. In F2 the ratio is 63 red: I white and in F3 it obeys the law as expected.

When four factors in the same direction are present, the ratio in F2 is 255 I; the writer considers it highly probable that this is the case with the ligule character in oats.

In the crosses of two lines, each of which possesses an independent factor operating in the same direction (Ab X a B), the offspring will include some combinations in which both factors enter and others in which they are absent. Through crossing two red-grained wheat lines which had exactly the same appearance, the writer oblained in F2 the segregation 15 red: I white and in F3 the further segregation as expected. (Berichte der deutschen botanischen Gesellchaft, 19, 1911, pp. 65-69):

In the same way the crossing of two very similar common oats, each of them possessing a factor for spreading branches (Ab X aB) produced in F2 and F3 the one-sided oats (ab) as well as forms (AB) with branches more divergent than in either parent, as expected.

In this manner apparent novelties, apparent mutations, arise.

The writer has found factors having the same direction in determined numbers, in a series of quite different characters of wheat and oats, not only in the colour (of the glumes in oats, of the ear and grain in wheat), but also in the size and form (length of the internodes in the ears of wheat, type of panicle in oats) (1), as well as in several other characters (presence of ligule in oats).

Factors in the same direction are often cumulative, that is, their effects add up, so that the character is more strongly impressed the greater the number of factors present. By means of several combinations of the factors, especially when heterozygotes are taken into consideration, a continuous variation of a hereditary rature may arise.

(1) The common oats possess several factors in the same direction for distant insertion of the branches; on crossing with Tartarian oats (Avena orientalis), the one-sided oats in F2 do not amount to a quarter of the individuals, but to a much lower proportion.

2

The above-described behaviour of the practically important characters (resistance to cold, rust, etc.), giving intermediate and transgressive segregation, is easy to understand by means of the theory of several factors working in the same direction and of a cumulative nature. Already with four factors, on crossing the extremes (ABCD X abcd) these extremes will be formed only once each in every 256 individuals; all the other 254 individuals are intermediates; in a limited number of individuals the whole segregation is intermediate. The individuals of medium character will be the most numerous, because the gametes with medium number of factors (two factors) are the most numerous. By means of the curve of combinations (Kombinationskurve) of several factors operating in the same direction the origin of a normal curve of frequency of hereditary variation is at least partially explained (1).

The transgressive segregation of practical characters after crossing two intermediate constant lines is to be attributed to the presence of several factors operating in the same direction in these lines, in the same way as the crossing of two similar red lines (Ab XaB) gives rise to partly deeper red (AB) and partly white (ab) as transgressions.

The principle of the action of several factors in the same direction represents, for practical breeding, the principle of the work of continuous combination.

In winter wheat, the object aimed at is to combine always in a better manner the probably many inner factors which determine the practical characters of productivity, resistance to cold, to diseases and to lodging, etc.; and little by little good progress has been achieved in the desired direction (see above), even if the chief aim of the work, namely to unite the productivity and other characters of the best West European varieties with the resistance to cold of the native Swedish varieties, is still far from being attained.

Further, an endeavour is made with both wheat and oats to obtain an increase in productivity by crossing two varieties, equally productive, which might eventually be the combination of several factors. In this direction some results have already been obtained, with winter wheat (Cf. Sveriges Utsädes forenings Tidskrift, 1912, pp. 317-318) and with oats (result not yet published).

The principle of continued combination work in practical breeding is not based only on the principle of factors operating in the same direction, but in general on the multiplicity of the inner factors which determine the exterior characters, and the complexity of the segregations caused by it. Even when only one factor working in a particular direction (e g. for black glumes in oats) is present and in F2 the segregation is 3 black: I white,

(1) By the term

Combination German writers (Baur, Schinz) mean a variation produced on the one hand by segregation of hybrids and on the other by new combination of "hereditary units ". Cf. BAUR, ERWIN: Einführung in die moderna Vererbungslehre, pp. 186-187. Berlin, 1911.

the segregation is by no means simple, because the black homozygotes may show very various shades of black, which points to the existence of modifying factors (reinforcing or inhibitory factors). A similarly modified segregation is very frequent in several characters of wheat, as well as of oats; the writer is conductnig special experiments for the investigation of this question. He has recognised quite special inhibitory factors in both wheat and oats (Zeitschrift fur induktive Abstammungs- und Vererbungslehre, 5, 1911, pp. 1-37), and it is very probable that these inhibitory factors also are not simple, but series of factors, and that there may be inhibitory factors operating in the same direction.

Considering this multiplicity of inner factors and the consequent possibility of improvement - though it may be slow and gradual—by the combination of these factors, the writer has come to the conclusion that together with the selection of already existing combinations, systematic artificial continuous crossing will in the near future play the most important part in plant breeding.

The writer has several times recognized spontaneous variations in homozygotic lines (Zeitschrift fur induktive Abstammungs- und Vererbungslehre, 5, 1911, pp. 1-37; Verhandlungen des Naturforscher Vereines in Brünn, 50,1911, pp. 139-156). These spontaneous variations are always due to the disappearance of Mendelian factors (mutations by loss), and nothing new has been produced in the cases hitherto observed. Nevertheless, the writer considers it not impossible that there may be mutations by loss which will prove valuable to breeders. For the investigation of this question also the writer has extensive experiments in course.

DETAILED PUBLICATIONS ON THE BREEDING WORK OF THE WRITER. NILSSON-EHLE. 1. Ueber die Winterweizenarbeiten in Svalöf in den Jahren 1900-1912, (Work on winter wheat carried out at Svalöf from 1900 to 1912). Beiträge zur Pflanzenzucht, III, pp. 60-86. Gives a complete list of the writer's 31 publications on wheat.

2. (Experiments in crossing oats and wheat). I. Lunds Universitets Arsskrift 1909, N. F. Afd. 2, Vol. 7, No. 2, pp. 122. Ibid., 1911, N. F. Afd. 2, Vol. 7, No. 6, pp. 84.

3. (Annual Reports on the work on oats and wheat at Svalöf). Sveriges Utsädesförenings Tidskrift, 1906-1912.

The Present State of Motor Cultivation in Germany

by

Dr. GUSTAV FISCHER,

Professor at the Royal Agricultural Higher School in Berlin.

Farmers who possess large extents of heavy soil can use to advantage the double-engine system of steam ploughing in which a balance plough is hauled by a wire rope alternately by each engine. The increase in the