The Botanical Review: March, 1938
M. J. Sirks
Genetisch Instituut, Groningen, Holland
In the play called Genetics which has been performing in theaters all over the world for almost forty years, the leading part in the beginning was played by the plant Pisum which a few years later was succeeded by Oenothera. Since 1910, however, a freshman has been entrusted with the role of chief character, the delicate fruit-fly Drosophila. This beginner proved to be one who knows his own mind, for he introduced a novelty by adding a chorus of eight chromosomes, and the character of the play, as a result, underwent a profound change.
It may seem at present that the entire problem of heredity is wholly governed by the action of these chromosomes and the genes which they contain, for most geneticists have been inclined to confine their attention to the role of these genes, while the plasm has been for the most part neglected. A sound construction of genetics, however, requires a study of the cell as a whole, in which the process of inheritance is located. Geneticists forget that the genes as such can show their effects in the phenotype only through the instrumentality of the plasm. It is fitting, therefore, that attention be drawn to a number of facts which point to a more or less independent action of the plasm as a determiner of or a contributor to inheritance.
A striking example of the influence of the plasm in inheritance and as revealed by zoological material, may be found in the studies of Boycott and his collaborators (1930) on sinistrality in the snail Limnaea peregra. The main results of a cross in this species between a pigmented dextral strain and an albino sinistral strain were as follows:
3 pigmented to
3 pigmented to
|F3||3 dextral to|
|3 dextral to|
The interpretation of segregations in these results, so far as they concern the albino factor, is not difficult, but the matter is not so simple with respect to the coil. On the basis of this preliminary results by Boycott, Sturtevant (1923) has assumbed that a dextral factor is dominant, while sinistrality may be regarded as recessive. This assumption, however, does not explain why the F1-generation from dextral mothers as a whole is dextral, that from sinistral mothers entirely sinistral. Some light is shed upon the problem, however, by the F3-generation which consists of so-called -groups, a mixture of three dextral broods to one sinistral brood. In this generation, segregation did not affect only certain individuals, but the broods as a whole, which were quite uniform with respect to the twist. This phenomenon finds an explanation in the assumption that the twist of the snail's shell is determined by the genotype of the mother and is fixed in the plasm of the latter's eggs. The nucleus of the zygote is thus without influence respecting this character in either the fertilized egg-cell or in the animal which develops from it.
This explanation is fully corroborated by the observations of Conklin (1903) who noted that the normal dextral twist in the snail Crepidula is fixed at a very early stage of development. In this case the first cleavage of the fertilized ovum is "dexiotropic" in dextral snails, i.e., both nuclei with their surrounding masses of plasm shift in both cell halves in clockwise direction, while in sinistral snails the movement is counter-clockwise. Conklin concluded from his observations that this difference between dextral and sinistral snails is rooted in the structure of the undivided egg-cell. It may thus be assumed that Boycott was correct when he described this peculiar behavior as "delayed inheritance" and not as "maternal inheritance."
Much less clear, though apparently similar in nature, is the situation in silkworms (Toyama, 1913; Tanaka, 1924) which probably may also be interpreted as the result of an after-effect induced in the egg-plasm by the genotype of the previous maternal generation.
Seed Color In Matthiola
As early as the first year of young Mendelism, in 1900, Correns found a difference between the reciprocal crosses of Matthiola incana with a blue embryonic epidermis, and M. glabra with a yellow embryonic epidermis. Seeds obtained from a cross of M. incana x M. glabra were as dark blue as those of the incana-mother, and those secured from the reciprocal cross showed a series of colors ranging from clear yellow to blue, but the darkest type of blue was of a much lighter shade than that of pure incana. The seeds produced by these F1 plants segregated as F2 embryos into 76.8% blue, from dark to light, and 23.2% yellow. The genotypes of all the F1 seeds were the same, but there was a difference between the plasm of M. incana and of M. glabra. That of incana immediately became dark by virtue of the presence of the heterozygous blue factor, while the glabra-plasm did not betray itself until long after the nuclear fusion, the reaction of this type of plasm upon the heterozygous blue factor being much slower than that of the incana-plasm.
Another example of plasmatic inheritance, in which not the plasm as a whole but special ingredients of it are involved, is found in the inheritance of chlorophyll. Chlorophyll is localized in the chloroplasts, and since the number of these bodies in the egg-cell is far greater than the number, if any at all, which is transmitted by the pollen-tube, it seems self-evident that at times the reciprocal crosses may be different with respect to chlorophyll production. Such cases have been found by deVries (1913), Renner (1924) and Krumbholz (1925) in the cross of Oenothera Lamarckiana x O. Hookeri, where the reciprocal hybrids between the homozygous O. Hookeri and the complex-heterozygous O. Lamarckiana produced different results. When O. Hookeri was used as pistillate and O. Lamarckiana as the pollen parent, the offspring in both types, Hookeri-laeta and Hookeri-velutina, was normal green, while the reciprocal cross with O. Hookeri as the pollen parent produced green laeta- but yellowish and very feeble velutina-individuals, most of which died young. Among these yellowish velutina plants, however, about 15% of the seedlings showed green spots on a yellowish background and appeared to be viable; the green spots enlarged considerably and the seedlings developed into yellowish-spotted or even green plants. On the other hand, among the green velutina plants of the reciprocal cross a few exceptions were found in which yellowish spots on a green background could be observed. Similar results were obtained by Dahlgren (1923, 1925) in the cross Geranium bohemicum x G. deprehensum, and by Noack (1931, 1934) in Hypericum acutum x H. montanum.
Renner (1924, 1929, 1934) explains this situation by assuming that the plastids of O. Lamarckiana are different from those of O. Hookeri. There appears to be no doubt that the pollen-tube of Oenothera transmits a few plastids, but the number is considerably less than that in the egg-cell. The diploid nature of the velutina individuals is produced by the fusion of the Hookeri nucleus and the velans complex, and it may be assumed that the Lamarckiana plastids are unable to develop chlorophyll under the influence of this nuclear combination, while the plastids of Hookeri are able to do so. Thus, a velans egg-cell of Lamarckiana fertilized by a Hookeri pollen-tube will contain a large number of plastids that remain yellow and a few which by way of exception develop chlorophyll. The reciprocal cross produces many Hookeri plastids developing into green chloroplasts and in a few cases also a few velans plastids which remain yellow. Michaelis' observations (1935) corroborate this hypothesis very satisfactorily.
This explanation by Renner is not accepted by Noack (1931, 1934) who believes that the plastids as such are not the primary cause of this type of variegation, but that some metabolic disorder is produced by a disharmony between the nature of the plasm and the genes contained in the nucleus. That this metabolic disorder should play a role in one cell and not in the other, is a problem which Noack's point of view can not explain.
Two series of researches have especially contributed toward a solution of the above problem. First of all, there are the classical experiments with mosses by Wettstein (1924-1935) and his students (Becker, 1932; Dörries-Ruger, 1929; Schmidt, 1931; Schwanitz, 1932; Tobler, 1932). Their material was exceedingly favorable for furnishing important and interesting results. By various methods Wettstein tried to obtain individuals with polyploid chromosome numbers, by anaesthetics, by applying low temperatures, and by regeneration. The last method has given by far the best results.
Parts of the sporogonium begin to develop protonemata on artificial media as the result of renewed cell divisions, and these protonemata, like the sporogonia themselves, are diploid, and from them diploid moss plants with diploid gametes can be grown. Fertilization by means of these diploid gametes is possible under special conditions, as between two diploids or between a diploid and a haploid gamete. In this manner tetraploid and triploid sporangia were obtained, from which triploid and tetraploid protonemata and moss plants could again be secured by regeneration. A complete series of haploid, diploid, triploid and tetraploid moss plants was thus established. Applying the process of regeneration to hybrids between different strains of the same species and to species-crosses, rich materials were acquired in which the plasm of one type could be combined with one or more sets of chromosomes of another, and in various combinations. Reciprocal crosses between various strains of the same species did not show any differences; those between species or genera, however, in most cases were more or less unlike.
A typical example from Wettstein's researches is the cross of Funaria mediterranea (Me) x F. hygrometrica (Hy). The species F. mediterranea possesses small sporocarps with a tall and acute operculum, leaves with filamentous apices, midribs in the leaves which end suddenly beneath the apex, and paraphyses which consist of spirally arranged oval cells. The other parent, F. hygrometrica, has large sporocarps with flat and broad opercula, leaves ending in a broad and pointless apex, midribs extending into the leaf apex, and paraphyses of one straight row of more or less spherical cells. The diploid sporocarps, produced by crossing these species, are different in both reciprocal combinations; those from Me x Hy are smaller with tall and acute opercula, almost like the mother type; those of HyMe are larger with broad and flat opercula, just like the hygrometrica mother. Sharp differences were found in the cultures of diploid hybrid plants, obtained by regeneration from diploid sporocarps. Particularly the length of the midribs appeared to be governed entirely by the mother type; in MeHy plants, this midrib ended far below the leaf apex, while in the reciprocal HyMe the midribs extended to the apices. Filamentous leaf apices were found only very slightly developed. The paraphyses of MeHy were spiral, those of HyMe straight.
Cultures of the haploid offspring, grown from spores, showed that this maternal influence could be observed for all three characters, though not to the same degree. The length of the midrib varied very little, but in all MeHy individuals it never reached the leaf apex. In HyMe it did so without exception. This maternal character was continued in all following generations. The form of the leaves did show a greater diversity and the proportions between length of leaf and filamentous apex were measured. The mean value of this proportion for pure Me-plants was 2.47, for Hy, 27.72. The diversity among the F1 haploids for both reciprocal crosses showed that the F1 segregational types in MeHy varied between 1.5 and 7.5, which figures were far from those of the paternal type; the haploid individuals in HyMe varied between 7 and 28 with an overwhelming majority in the 27-28 class. For the paraphyses the segregation appeared to be of greater importance, though a maternal influence could not be denied.
The only possible explanation of these phenomena lies in the assumption that the maternal plasm plays a role in the development of these characters. Sterility was of no importance, for MeHy produced only 3% of sterile spores, and only 5.8% of the young plants died; in HyMe the sterility was 0.32% to 3.81%, and the lethality in young plants was 5.7% The number of cultivated individuals was sufficiently large to show all possible segregations, 450 in MeHy and 543 in HyMe. The only possible explanation appears to lie in the admission of plasmatic influence.
This conclusion has been corroborated by Wettstein in a continuous series of back-crosses in which Me-plasm was combined with pure diploid nuclei of the Hy-type, and otherwise. Such back-crosses were repeated in some cultures more than eight times, so that finally it was certain that the plasm belonged to one species and the nucleus entirely to the other. There seems to be no doubt that in this case the plasm possesses inheritable characters by which it is able to react in its own way to the activities originating from the nuclear genes, or even to produce certain characters entirely by its own powers. On this ground Wettstein has applied the term "plasmon" to the "genetical elements of the plasm," as contrasted with the term "genom" by which Winkler had denoted the whole collection of genes contained in the chromosomes.
The above-mentioned three characters of moss plants seem to point to different intensities of this plasm, but it may be assumed that in every case plasmon and genom are struggling for ascendency. As to the length of the midribs, the plasmon is highly "antecedent," while the influence of genes disappears entirely; for the filaments of leaves this antecedence is somewhat more feeble so that the "recedent" genes may do their work in a small degree, and the form of paraphyses is caused by some balance between plasmon and genom.
The other series of reciprocal crosses which has promised to be of importance in the problem of plasmatic inheritance is that between species of the genus Epilobium (E. hirsutum, E. montanum, E. palustre, E. parviflorum, E. roseum). These studies were started by Lehman (1918-1936) and his students Koehler (1929), Schwemmle (1927), Schnittger (1932), Graze and Schlenker (1936) and Hinderer (1936). The work was soon followed by that of Renner and Kupper (1921) and with most success by that of Michaelis (1929-1935) and his students, von Dellingshausen (1935, 1936) and Wertz (1935). The crosses in which E. hirsutum and E. parviflorum were used as one of the parents, showed important differences between the reciprocal hybrids, in total size, length and width of the leaves, size of buds, length and width of petals, length and width of ovaries and ripe fruits, fertility of pollen and embryos, and in intensity of the curvature of stem apices. Though at first some resemblance to the reciprocal differences between various types of Oenothera was surmised, Renner and Kupper (1921) soon assumed that no such analogue was possible, but that a difference between the plasms of both plants was the real cause. A most conclusive proof of this assumption has been produced by Michaelis. He applied the method of continuous back-crosses which was originated by Wettstein in his studies of mosses, to the cross E. luteum x E. hirsutum, and in this way the hybrid was back-crossed fourteen times with the pollen parent E. hirsutum. For this back-cross Michaelis has given the formula ((((L x h) x h) x h) x h) and so on, or Lhn, in which n indicates the number of generations obtained by back-crossing. By means of calculation and by experiment it is possible to prove that beginning with the eighth back-cross generation, the nucleus in these back-cross individuals was a purely homozygous hirsutum-nucleus.
These Lhn plants possess all the morphological characters of real hirsutum-individuals, with which they are almost identical, a fact which proves that these characters are all determined by the nuclear genes. However, the cross of these Lhn plants with E. luteum does not produce any reciprocal differences, while the cross Hhn x E. luteum shows a great number of such differences. This proves in an irrefutable manner that these reciprocal differences are due to the plasm and that the characters of this plasm, even in combination with a homozygous foreign nucleus, have on the whole remained the same.
Such plasmatic differences have been acknowledged by a great many authors as existing between higher taxonomical units, as families and genera, but the work of Lehman and his collaborators has shown that such differences exist also between biotypes within the species E. parviflorum, while Michaelis established the same condition for strains of different origin within E. hirsutum. The subspecies of Vicia Faba, V. major and V. minor, do show analogous plasmatic differences, according to my own studies (Sirks, 1931a-c, 1932), and Schlösser disclosed a similar sitation in varieties of tomatoes (1935).
Finally, it may be mentioned that a study on the inheritance of dichasial and sympodial growth habits in Datura, publication of which is in course of preparation, has yielded further evidence of plasmatic influence in heredity.
In addition to the foregoing series of researches on mosses and Epilobium, a number of other cases appear worthy of mention, in which plasmatic differences have been detected. They may be grouped under three headings:
In Wettstein's studies on mosses it was found, as mentioned above, that the midribs in MeHy-crosses never reached the leaf apices, while in HyMe-plants they did so without exception. It may be assumed, therefore, that this character is largely if not entirely governed by the plasm. In a similar manner Schwanitz (1932) has proved that the regeneration of protonemata is entirely under the influence of the plasm. His data indicate that a genuine case of plasmatic inheritance can not be denied in this case. As we shall later note, osmotic values probably play a role in this instance.
A similar situation with respect to osmotic values has been demonstrated by Schlösser (1935) in different strains of the wild species Lycospersicum esculentum. The values were measured by the cryoscopic method (Walter) with pressed juice. Under dry glass-house conditions they gave 6 atmospheres for the strain U1, 8.3 atmospheres for U2. The reciprocal crosses between the two strains in both F1- and F2-generations behaved just like the mother strains.
In these experiments no influence of any genes could be observed, though it does seem questionable that such an influence should be entirely absent. Two possibilities may be considered in this case: 1) the parents may be different in plasmatic nature, but their genotypical structures identical, and, consequently, segregation of genes does not take place; 2) the antecedence of the plasmon may be of such a strong nature that the recedent genes remain unobservable.
A great amount of research has been concerned with the difference in action by different plasms on the same heterozygous or homozygous genotypes. The characters involved, without exception, are of a quantitative nature; thus far, no plasmatic differences concerning qualitative characters have been found. It is very fortunate that these quantitative characters can be measured in a very exact manner; curvatures of stem apices, as those found in so many crosses of Epilobium, do not lend themselves to such measurements as do the dimensions of stems, leaves, flowers, fruits, etc. A very clear-cut example has been discovered in my crosses between the two subspecies of Vicia Faba, namely, major and minor (Sirks, 1931b, 1932). These crosses showed that for dimensions of stems, leaves and fruit, one series of quadruple allelomorphs was fundamental in controlling growth (G1-G4), while in stems there was a series of three allelomorphs as internodal factors (I1-I3), and in leaves two additional factors (B and T) for leaf length and one (W) for width. Among the crosses studied, two may be mentioned in particular. Crosses 24 and 24a were reciprocal between a major G2wBTI3 and a minor G3wBTI3, thus differing for length of stem in the G factor only, for width of leaf in G and W, for fruit length in G, B and T. From my data (Sirks, 1932) the conclusion may be drawn that the plasm in major- and minor-plants is different in the sense that the minor-plasm, when compared with the major-plasm, reacts upon the same genotype for G factors: 1) more strongly as far as stem length is concerned; 2) in an equal manner for leaf dimensions; and 3) with diminished intensity for the phenotype of length of fruit.
Analogous data for size of leaf and plant have been published by Schlösser (1935) in his tomato studies. In certain strains, as mentioned above, Schlösser found osmotic differences in the plasm, and these strains in particular have given rise to the conclusion that genes for size are enabled to put forth their powers better in a plasm of lower osmotic pressure than in one of higher value. Possibly the explanation for the differences observed in Vicia relating to plasmatic reactions in the stems, leaves and fruits may be sought in the same direction. It may be that the osmotic value is subject to changes during the ontogenetic development of the individuals, and that in this way the differences between major- and minor-plasms can be accounted for.
Another phenomenon, observed in Phaseolus (Sirks, 1938), seems to point in the same direction. A plant had formed three pods at the basal part of the stem, the size of two of them being much smaller than that of the third and more typical pod. The seeds formed in these three pods were sown separately and seemed to indicate complete inheritance of the aberrant sizes of pods. From the third generation on (by selfing), a change could be observed; the mean values of these aberrant families approached closer and closer to those of the normal large-podded type; the normal dimensions for type II were attained in the fifth generation, and for type I, the smallest, in the eighth generation. Crosses between the three types showed similar results; at first, plasmatic inheritance of the smaller size seemed obvious, but in a few generations the normal size of the pod was entirely restored and the aberrant characters disappeared.
Interesting studies on the production of growth-substances by various strains of Epilobium hirsutum, the strains differing in plasmatic nature, have been undertaken by Lehmann's students, Hinderer (1936) and Graze and Schlenker (1936). Their results seem to point to some connection between the type of plasm and the intensity of production of growth-substances, but the results so far obtained are not entirely clear.
It is not at all surprising that plasmatic influences which possibly are rooted in physiological differences of the constitution of the plasm, are also observed in physiological phenotypical characters. Michaelis has found (1935) differences in branching and in resistance against mildew. Plants of Hhn and of Lh10 were grown in small pots, and under these conditions, involving deficiency of food, a peculiar difference in resistance toward Ersyphe could be noted. E. hirsutum is very susceptible to this fungus, while E. luteum never showed any symptoms of the disease. Though quantitative measurements of the intensity of disease were impossible, the difference between the susceptible Hhn plants and the resistant Lh10 individuals was so clear that the presence of a plasmatic influence could not be questioned. The genotype, however, assists the plasm in some manner in this resistance (cf. fig. 2 in Michaelis, 1935c).
As already mentioned, Schlösser has found plasmatic inheritance for osmotic values, without the apparent influence of any genes. It seems, however, that this may be an exceptional case. For mosses Becker (1931) observed that the number of chromosome sets expresses itself in the osmotic values, and the conclusion may be drawn from his data that an increase of genes diminishes the osmotic values in a regular sequence: Pi > Pi2 > Pi3 > Pi4 and Hy > Hy2 > Hy4, while addition of a foreign chromosome set interferes to a lesser degree than increase of chromosomes which belong to the species: PiHy > Pi; PiHy2 > Pi2Hy > Pi3; PiHy3 > Pi2Hy2 > Pi3Hy.
Studies on permeability have been published by von Dellingshausen (1935) with urea, potassium chloride, glycerine and succinimid in the Hhn and Lh10 strains of Epilobium. Her data indicate that for urea alone no plasmatic influence could be ascertained but that preliminary treatment with chloralhydrate caused a decrease in permeability in the Hhn plants, while the Lh10 plants were not affected. In all her experiments an influence was apparently exerted by the plasm. Comparisons between Hhn, Lh10 and Lln with glycerine activity in addition to the plasmatic influence.
Von Dellingshausen (1936) has continued her physiological studies on this material by measuring the viscosity of the plasm. Two methods have been employed, namely, a study of the outline of the plasm after plasmolysis (2, 10, 20 minutes), and a study of the influence of centrifuging and of the resulting displacement of nuclei and chloroplasts. These studies, conducted under normal conditions after preliminary treatment with chloralhydrate and under different temperatures, all showed that the viscosity of the luteum-plasm is less than that of hirsutum, that chloralhydrate increase the viscosity of hirsutum-plasm but diminishes that of luteum, and that decomposition of starch in Lh10 plants is more intense than in Hhn plants.
The results discussed above seem to show that in a number of cases the plasm plays an important part in the production of the phenotype and that phenotypes indeed are produced by the combined influence of the genotype and the plasm. A further problem involves the question as to whether or not the genotype can be influenced in any way by a foreign plasm. It has been demonstrated that such an influence may be brought about in the following four ways:
a) Zygotic lethality. The crosses between the two subspecies of Vicia Faba, major and minor, have produced clear-cut cases of zygotic lethality in minor-plasmatic heterozygous plants (Sirks, 1931a, 1932). In a number of reciprocal crosses between these two subspecies, the zygotic lethality in minor-mothers was much higher than in the parents or in the crosses with major-mothers. The data secured have shown in a very clear manner that the plasm of minor-parents is the cause of a significant elimination of zygotes, which elimination can be estimated to be about 25%. By means of a number of back-crosses between minor-plasmatic F1-plants and pure minor-pollen parents, it could be proven that there was no lethality of egg-cells, for the number of non-developing egg-cells in these back-crosses never surpassed 8%. This zygotic elimination was coupled in the F2 rather constantly with a number of genes which were heterozygous in the F1-plants.
In these crosses, the dominant genes, A, E, M and O, and the recessive ones, b and t, came from the minor-parents; the recessives a, e, m and o, and the dominant B and T from the major. The data plainly indicate that the homozygous combinations of genes, supplied by the major-parents, were not viable in plants with the minor-plasm, and thus the double recessives, aa, ee, mm and oo, were eliminated, which resulted in what appeared to be non-segregation of these factors; the same situation obtained with respect to the double-dominants, BB and TT, and here a 2:1 segregation in the minor-F1-plants was the result. These factors, A, E, M, O, b and t, belonged to one linkage group, as was revealed by their cross-over percentages.
The most plausible inference from these facts is that the combination of two major-chromosomes, supplied by the father, is not viable in minor-plasm, so that all zygotes containing this pair of chromosomes do not develop.
This zygotic lethality is not always expressed at such an early stage of development, and the effect may be postponed until a later age of the plants. In crosses between two varieties of flax which differ considerably in habit, "procumbens" versus "tall," Bateson and Gairdner (1921) observed a sterility of anthers in one of the reciprocal crosses, observations which have been continued by Chittenden and Pellew (1927) and Gairdner (1929). Procumbens x tall produced in the F2 a segregation into 75% normal individuals and 25% plants the anthers of which were shrivelled and did not contain any viable pollen. The reciprocal cross, tall x procumbens, delivered normal F2-individuals only. A series of back-crosses (Gairdner, 1929) has produced evidence that this "male sterility" depends upon a recessive m factor which is supplied by the "tall" parents, but which acts only in "procumbens"-plasm. Thus the MM-, Mm- and mm-individuals with "tall" plasm are quite normal and produce fertile pollen; in procumbens-plasm MM- and Mm- plants are normal and their pollen viable; but mm-plants showed male sterility. These results can be interpreted in this way, that mm-individuals in procumbens-plasm are hindered in their normal development, which hindrance finds expression only in the anthers. The active sterilizing influence of the type of the plasm is here retarded.
b) Gametic lethality. Not only zygotic lethality may be caused by certain plasmatic influences, but evidence is at hand that special types of gametes may also be lethalized in certain plasmatic environments. Skalinska (1928, 1929, 1930) has published studies on crosses between Aquilegia chrysantha and A. flabellata in which a number of genotypical combinations corresponding to those supplied by the pollen parents failed. The F1-plants showed an important gametic sterility, and it may be concluded from her work that gametes possessing a genotypical structure which resembled that of the original father were eliminated.
A similar result has been obtained in a variegated type of Vicia Faba (Sirks, 1931c) which could be ascribed to the presence of a dominant V-factor. The inheritance of this character is complicated by the fact that variegated plants sometimes develop plain green branches or a few green plants among their offspring. A study of the offspring from such green branches and green plants have shown, however, that this plain green is only phenotypic, while their genotypes appear to possess the V-factor in a heterozygous state. Variegated branches produced only variegated offspring, mixed with a few genotypically variegated but phenotypically green individuals, and phenotypically green branches or plants segregated their offspring into 1 genotypically variegated: 1 genotypically green plant. It may be assumed, therefore, that in individuals with the Vv-genotype, the type of plasm in the determining factor for the constitution of the offspring; in green plasm the dominant factor V is eliminated in one of the sexes (offspring Vv + vv), while in the variegated plasm the recessive v is discarded by the egg-cells or pollen (offspring VV + Vv).
Though not quite conclusive, some results of Honing (1931) seem to point in the same direction. He crossed Canna indica (with red-edged leaves) with Canna aureo-vittata (without red edges) and made back-crosses of both reciprocal combinations with C. aureo-vittata. Those back-crosses in which C. indica was the original, produced an excess of plants with red-edged leaves, while in those back-cross-generations in which C. aureo-vittata was the original mother, a shortage of this type of plants was observed. It may be assumed, therefore, that in aureo-vittata-plasm gametes or zygotes containing the dominant red-edge factor are eliminated, while in indica-plasm this seems to be the fate of gametes or zygotes containing its recessive.
c) Influence of the plasm on chromosome conjugation. A few additional researches into the influence of plasm on nuclear phenomena may be mentioned. Oehlkers (1935) studied the action of temperatures upon the chromosome rings and closed bivalents of Oenothera. He considered the flavens-type from the cross O. suaveolens x O. Hookeri, and its reciprocal. The flavens-plants normally combine their chromosomes into a ring of four and five closed bivalents. The total number of bindings in such a nucleus being 14, the percentage of open bindings under various temperatures could be calculated. In four of the results the differences between the percentages were greater than the standard error, so that it may be considered ascertained that suaveolens-plasm, as compared with Hookeri-plasm, increases the frequency of open bindings, and that extreme conditions intensify these plasmatic differences.
d) Influence of the plasm on the frequency of artificially produced gene mutations. A study by Stubbe (1935) has provided us with some data concerning the influence of the plasm on the frequency of artificially secured gene mutations. Röntgen radiations applied to Epilobium hirsutum and Epilobium Lhn (luteum-plasm with hirsutum-nucleus) produced a number of gene mutations, and from the data obtained it may be concluded that a definite dose of Röntgen rays provokes in foreign plasm a higher frequency of gene mutations than the same dose does when applied to hirsutum-nuclei in hirsutum-plasm.
Finally, there remains the problem as to whether or not a plasm can be changed under the influence of a foreign genom. Two opposed opinions on this question have been put forward. Wettstein's studies on mosses have led to the conclusion that a foreign nucleus is unable to change the nature of the plasm, and Renner agrees with this view-point. On the other hand, Lehmann and Honing (1930) take the opposite view. In a number of papers, Michaelis has tried to reach a decision between these two possibilities, and in his recent study on the plasms of Epilobium hirsutum, E. luteum, E. montanum, and E. roseum (Michaelis und Wertz, 1935) he gives the following conclusions: 1) The plasm of luteum transmitted its characters during eleven generations, though influenced by a hirsutum-genom; 2) It has been ascertained that the plasmon can be changed, though only slightly, by a foreign genom in the sense of a "Dauermodifikation" (permanent modification); 3) The male nucleus can transmit certain maternal characters to a slight degree. It seems to me that these conclusions from Michaelis clearly show that the main question, namely, Can the plasm be permanently changed by a foreign genom?, is still unsolved.
It may be said in conclusion that the question of plasmatic inheritance is becoming of greater importance, not only from a theoretical point of view, but also as it concerns applied genetics. The publications by Bittner (1936), Korteweg (1936) and Murray and Little (1935) show that in the problem of cancer, the plasm certainly plays a role in the inheritance of this disease. First of all, however, it must be emphasized that inheritance is not a matter of genes or of plasm alone, but is a problem involving both genes and plasm.