Popular Science Monthly/Volume 67/June 1905/The Mutations of Lycopersicum
|THE MUTATIONS OF LYCOPERSICUM.|
DURING the years 1901 to 1903 inclusive I published results of my observations and experiments concerning the horticultural variability, atavic reversion or degeneration, and phylogenetic mutation of the common cultivated tomato. The reader is referred to those publications for such statements of pertinent facts as may be omitted from this one. The object of the present article is to give in popular form a concise restatement of my experimental observations upon some remarkable cases of saltatory plant mutation and varietal changes of the tomato fruit, together with figures and additional discussions. Although the cases of plant mutation referred to constitute the leading part of my subject, I will first discuss the origination and decadent extinction of the improved fruit varieties which have arisen in connection with, and apparently as a result of, horticultural conservation. These discussions are necessary to the making of a clear distinction between fruit variation and plant mutation as I shall have occasion to refer to them.
The enormous increase in the importance of the tomato as an article of food during the past thirty years has so stimulated its cultivation that very many fruit varieties of fine quality have resulted, figures and descriptions of the more important of which are annually published in seed growers' catalogues. During that time also at least two new specific plant forms have suddenly originated by mutation from the common species, Lycopersicum. esculentum, making not less than three species of the cultivated tomato. It is desirable to characterize these species briefly in connection with the discussion of the fruit varieties which they bear. The two new species referred to I have called L. solanopsis and L. latifoliaium, respectively, of course leaving the original name, L. esculentum, with the unmutated, or mother form. This mother form, even as delimited by the mutations just mentioned, is much the most common one in cultivation. Its early stage of growth is fairly represented by the accompanying Fig. 2. The form to which the name L. solanopsis is given is well represented in a similar stage of growth by Fig. 1. Short descriptions of these two forms are recorded on a following page. I am not now able to present a figure of L. latifoliatum, but it is represented by the plant which bears the fruit variety known to gardeners as the Mikado, or Turner's hybrid. I think the latter name, when applied to the plant, is misleading because this specific plant form doubtless originated by true mutation, as L. solanopsis has done; and it is by no means certain that even the fruit variety which it then bore originated by hybridization.
|Fig. 1. Representing L. solanopsis, the Daughter Form.||Fig. 2. Representing L. esculentum, the Mother Form.|
The difference of L. latifoliatum from the two other forms mentioned is conspicuously seen in its peculiar foliage, the leaves having decurrent petioles and broad, flattened leaflets with their borders entire instead of notched or crenulated. These three species are as well defined and distinct as are any others of the dozen recognized species of Lycopersicum, and as distinct as are many of the recognized species of other plants, whether wild or cultivated, and there is apparently no tendency of the two derived species to revert to the mother form. I do not know of any case of hybridity between any two of them, and no indication of further mutation of the two new species has been observed.
It is from these three specific plant forms that the improved fruit varieties have arisen. The greater part of them have arisen from L. esculentum, as that species has been delimited; a considerable number have arisen from L. solanopsis, while L. latifoliatum has hitherto shown the minimum of varietal change in its fruit. These varieties differ more or less from one another in shades of color, but their valuable qualities appear in the increased solidity or consistence, size and fine flavor of the fruit. Their origination has, in many cases at least, been sudden and fortuitous; and, although their specific characters are usually heritable, they are so peculiarly subject to degeneration that they hardly possess racial properties. The causes of their origination and extinction appear to be irrelative, and both are independent of specific plant mutation. Still, as I shall show, a few cases of coincidence of their origin with plant mutation are known; and more or less of common plant variation, and even of dwarfing, have sometimes occurred coincidently. Moreover, all cases of atavic reversion, or degeneration, of fruit varieties from their fine quality have apparently been unaccompanied by any material change in the plants bearing the degenerating fruit. Frequently special reference to those reversions will be made further on, but I will first turn aside for a few more or less theoretical remarks upon the character and manner of origination and extinction of the fruit varieties.
In the simple cases of origination of the fruit varieties of the tomato, unaccompanied with plant mutation, the change occurs only in the pericarp, which becomes the fruit. Also, when atavic reversion occurs it is only the pericarp, that is, the fruit, which is materially affected; and both kinds of these changes are doubtless of molecular origin in the germ cells. Likewise, in all cases of phylogenetic plant mutation the initial act is molecular, and occurs in the germ cell of each ovule which gives origin to a new plant form. There is no apparent reason to doubt that the mutative acts which thus respectively produced the two new specific forms from L. esculentum might have occurred without the coincident production of a new fruit variety, but as a matter of fact a new fruit variety of high quality was coincidently produced in each case; and the coming change in both plant and fruit was doubtless initiated in one and the same germ cell. Still, because of the essential difference between plant mutation and fruit variation, I think this pericarpal change was only a varietal coincidence, and not an essential part of the phylogenetic process in those cases. Moreover, that change is known to have occurred separately in other cases, and to have resulted only in weakly heritable fruit varieties.
Known cases of degeneration, and final extinction of fine fruit varieties as such, are somewhat numerous, for their instability is very great, although at least most of them remain true to seed from year to year under favorable conditions. Whether the varieties which have arisen from the two new specific plant forms that were derived by mutation from L. esculentum are more permanent than are those of the mother species I do not now know, but all the cases of atavic reversion known to me have occurred with fruit varieties of the mother species. It therefore seems possible that the fruit varieties arising from L. solanopsis and L. latifoliatum may be less liable to hybridity, or otherwise more permanently heritable, than are those arising from the mother species, L. esculentum. The fruit of the two derived species has always been of fine quality and, for them, intraspecific fruit reversion would not be degeneration; but the original fruit of the mother species was very inferior, and any reversion of its improved quality would be degeneration. For those who may have the opportunity, it will be interesting to observe the relative permanence of the fruit varieties arising from the different species, and the course that may be taken in any qualitative changes that may occur in the fine fruit varieties of the two derived species. In the case of the mother species the trend of fruit degeneration is direct, intraspecific and towards the primitive fruit condition. If similar reversional trend in the fruit of the two derived species could occur it would necessarily be accompanied by coincident reversion of specific plant characters, an occurrence which I think improbable, or we should have one and the same kind of degraded fruit borne by different species.
Eeturning to the practical consideration of atavic reversions of fine fruit varieties, it may first be mentioned that they often occur locally and affect only a few plants, or the crop of a single garden or field, the variety thus affected remaining unchanged elsewhere, but, as I shall show, cases of reversion are often much more extensive. The progress of reversion is sometimes slow and sometimes sudden, the whole change in the latter case often occurring in a single generation. The effect is much the same whether the degenerating process is sudden or slow; and however widely the improved varieties may have differed from one another, the reversional trend of all is towards the comparatively small globular berry that may be properly regarded as the primitive tomato fruit form. In slow degeneration the fruit begins to ripen unevenly; it diminishes in size and becomes comparatively soft, and has a rank taste. The walls and dissepiments become thin, the seed compartments are reduced to four, three, and even to two, and the seed pulp is more abundant and more watery. In sudden reversion the primitive berry condition is reached, or approached, at a bound. These remarks concerning degeneration apply especially to the varieties which have arisen from L. esculentum, as delimited, of which cases only I have had personal knowledge.
With the possible exceptions which were merely suggested in a previous paragraph, the duration unimpaired of any of the highly improved fruit varieties of the tomato is only a few years under favorable conditions; and constant care is necessary to maintain their fine quality. Therefore the tendency to deteriorate is doubtless inherent; but this result is evidently hastened by careless cultivation, repeated planting of the same ground without rotation with other crops, the growing of plants from unharvested seed, cross pollination with inferior kinds, differences in character and fertility of the soil, and the influence of a climate much warmer than, or otherwise different from, the one in which the seed was produced. That this fruit degeneration is sometimes slow and sometimes sudden; that it is imminent and variously excited to action; that it is not confined to sporadic cases of single plants, but may, and often does, equally affect a whole crop, and sometimes all the crops of a wide region, is shown by the following statements of relevant facts.
Every person who habitually visits the vegetable markets of any one of most of our towns and cities which are supplied from neighboring gardens is familiar with the different grades in quality of the tomatoes there on sale. Indeed, it is often easy to recognize among them different stages of reversion from some of the more common improved varieties, notably the Acme. These are too plainly cases of gradual degeneration, resulting from careless cultivation and crossing with inferior kinds, to need explanation. Several of my correspondents have furnished me with important corroborative facts. Dr. Geo. G. Groff writes that he has for many years observed in central Pennsylvania, that tomato plants which sprang from seed of good varieties left in the ground during the winter always produced inferior fruit, usually the small kind called cherry tomatoes. Miss Mary E. Starr informs me that during her residence in Saint Martin's Parish, southern Louisiana, her father found it necessary to procure tomato seed from the north for every crop grown on his plantation, because the seed from even the first crop of tomatoes grown there usually produced very small and inferior fruit. Mr. L. S. Frierson, however, writing from northwestern Louisiana, says that he has produced excellent fruit, true to seed, from his home-grown crops. Mr. H. J. Browne, of Washington, D. C, sent me from a plantation near Havana, Cuba, a small parcel of cherry tomatoes taken from plants which he found growing there luxuriantly. The planter assured him that they were the immediate progeny of the first Cuban crop of a fine large fruited variety, the seed of which he obtained from New York under the well-known varietal name of Trophy. He also asserted that such degeneration was always the result of his attempts to raise tomatoes from Cuban-grown seed, however fine might be the variety from which his original seed was obtained. The fruit of the first Cuban crop, like that of southern Louisiana, was always true to northern seed, showing that the initial step towards atavic change took place in the germ cell of every one of the first seeds produced on those southern soils, and that the reversion was therefore sudden and aggregate. Mr. Browne, who has business interests in Calapach island, which lies thirty miles east of the Isle of Pines and eighty miles south of Cuba, also informs me that there are now growing on that island tomato plants which are four or five years old, they having changed from the condition of annual, to that of perennial plants in that tropical climate. Furthermore, the fruit of those plants has changed from a good variety of large size for the first fruitage to the cherry form and size before mentioned for the later fruitages.
These credible facts, gathered from widely different sources, plainly indicate that various exciting causes of varietal fruit degeneration exist, but they throw little light upon the real nature of those causes. The facts mentioned also indicate that many new opportunities are likely to arise for scientific agricultural experimentation with the tomato. Our tropical and subtropical island possessions will doubtless soon be called upon to supply, for our own and other countries, the increasing demand for early tomatoes, just as northern Egypt has been made the early tomato garden of Europe. My present object in referring to these facts, however, is their application to the second part of my subject.
This second part pertains to phylogenetic plant mutation as distinguished from ordinary plant variation and the production of new fruit varieties. The immediately following remarks embrace in narrative form an account of two cases of saltatory plant mutation which have fallen under my experimental observation. In the spring of 1898 I purchased a couple of dozen young tomato plants of the Acme variety which had been germinated by a gardener near Washington, D. C, and transplanted them, before any of their flower buds were formed, in a garden plot of a few hundred square feet, upon my house lot in the city. Short specific descriptions of these plants and their progeny are given for the purpose of showing their differences.
As the plants matured and fruited they were found to possess all the characteristics of the Acme variety, and of typical L. esculentum. They early became decumbent, and at full maturity they were large and diffuse; the haulms, which were slender and somewhat numerous, reaching a maximum length of more than two meters; color of the foliage a comparatively light green; the petiole-midribs long and slender; leaflets moderately narrow, distant, petiolulate, and their surfaces only slightly rugose; fruit of moderate size, usually depressed-globular in shape, but sometimes transversely oval, uniformly ripened, fleshy and well flavored, and in ripening the chlorophyl green changed to a deep crimson through more or less of yellow.
I selected seed from the fruit of the best plants of this crop of 1898, cured, and sealed them in a packet, and planted a random portion of them in my garden in 1899. I expected to produce true Acme plants from this sowing because of the well-known comparative stability of that variety and of my care in selecting and preserving the seed; and also because no other tomato plants were grown with them or in the same neighborhood, from which cross fertilization might have occurred. To my surprise, however, all the plants which grew from those seeds were distinctly different from the parent plants, not only as to fruit, but as to specific details of plant structure; and they were all alike in those characteristics. It may be incidentally mentioned that a difference was recognizable in the earliest stage of growth of the plantlets; the cotyledons were proportionately short, placed low on the stem, and in a goodly number of instances, triple; a character which I have never observed in any other tomato plantlets. At maturity the plants were sturdy and compact, standing erect with little support until after the first fruits were visible, and reaching a mature length of only about two thirds of that of the parent plants; haulms strong and comparatively few; foliage dark green; petiole-midribs short and strong; leaflets moderately broad, not distant, sessile or nearly so, and their surfaces strongly rugose; fruit similar to that of the parent plants in size, shape and consistence, but of finer flavor and more delicate in color, changing from a dark chlorophyl green to cherry red or light crimson through a neutral or flesh color, and not through yellow. I preserved no seed from this crop of 1899, and supposed the fruit variety was therefore lost, as indeed it was, but two years later I recovered it, as will presently be shown. This fruit differed considerably from any other of the numerous varieties known to me; but the plants had essentially the same specific characters as those which had previously been produced by gardeners, known as the potato-leafed variety of tomato. It was by those characters that I designated L. solanopsis as a distinct species.
In the spring of 1900 I purchased from a Philadelphia company of seed growers a packet of their 'selected Acme tomato seed' which was grown in 1899 on a Pennsylvania farm, more than a hundred miles from the place where my first Acme plants were grown. From this seed I grew thirty plants to maturity, every one of which, with its fruit, was true to the Acme variety as I have just described it for my crop of 1898. Fig. 2 represents one of those plants as it appeared in the early stage of its growth. Its smaller size than Fig. 1 is due only to the relative size of the growing plants at the time the photographs were taken. The conditions of cultivation in this case were identical with those in the former case; no other tomato plants were grown with them, nor were any grown in the neighborhood; and the resulting crop of fruit gave no visible indication of impending mutation. I as carefully preserved seed from this crop as I had done in the former case, and planted them in my garden in 1901, believing that I should produce Acme plants, notwithstanding my former experience. On the contrary, the result was an exact duplication of my experience with the crop of 1899, every plant and every fruit partaking fully and uniformly of the duplicated mutation. The plant description, including that of the fruit, which is given in the immediately preceding paragraph applies as exactly to the plants of the crop of 1901 as it does to those of the crop of 1899. Fig. 1 represents a plant of this crop in the early stage of its growth, when it was beginning to shed its first flowers. Its deeper shade of green adds to the difference of aspect between the mother and daughter forms.
The Figures 1 and 2 are copies of photographs which were taken of the plants as they were then growing in my garden. The plant represented by Fig. 1 bore the new variety, which I have called the Washington. That which is represented by Fig. 2 bore the Acme variety. Although it can not be proved that the particular plant which is represented by Fig. 1 actually came from a seed borne by the plant represented by Fig. 2, I do not hesitate to assert positively that the plant form represented by Fig. 1 is the immediate progeny of the form represented by Fig. 2. I make this statement with all the more confidence because all the work of my garden has been done con amore by my own hands, including the planting of the seed, the plucking of the fruit from which the seed was taken, and the curing and preserving of the seed for the next year's planting. In all this work I practised the same conscientious care that I have done in all my other scientific work in other fields. No tomato seed other than that which I have mentioned was in my possession during all the time my experiments were in progress, and I do not admit the possibility that any other seed was at any time substituted. Even if there had been any such substitution, it would not account for the mutations which I have described, which were phylogenetic in character and not the result of hybridization. The fruit of the mutated plant species was also a new variety and would not, in any ordinary case of germination, have been produced by seed of any other variety previously known. This new variety is as distinct as are any other fine varieties, and it has been true to seed every year since its origination. If my Acme plants, in either of the cases mentioned, had received adventitious fertilization by pollen from any other flowers than those of their connate crop associates, the cross fertilization would certainly have been incomplete as to the whole crop and various as to the kinds of hybrids produced. Even if it were credible that the first case of complete aggregate mutation was due to fortuitous cross fertilization from some unknown source, it would still be too much to believe that exactly the same hybridizing process should have been repeated in the same manner in a following year. It may be added that there is now much reason to doubt that hybridization, although always imminent among tomatoes, has ever been so effective an agent in producing improved varieties of either plants or fruit as has been generally believed. Indeed,' saltatory mutation and racial variation have doubtless produced many of the results among plants that have been attributed to hybridization; although the latter has produced many wonderful results.
At the close of this narrative of experimental observations it is well to call special attention to the assumed fact that the mutative process which produced the new plant form that has been described was essentially separate from the accompanying process of fruit variation, although the two processes were intimately associated in both their origin and development. The plant mutation was from L. esculentum to L. solanopsis; the fruit variation was from the Acme to the Washington variety. The new fruit variety which accompanied the new plant form is of fine quality and therefore of horticultural value; but the origination of any fruit variety is, from a naturalist's point of view, of far less importance than the origination of a species. Plant mutation produces species which are real entities. Fruit variation is limited to changes in the pericarp; and the most improved and heritable fruit variety thus produced may, by degeneration, become disassociated from the plant entity without any impairment of that specific condition.
There are two extraordinary features of the foregoing narrative of my observations. One of them relates to the sudden and complete mutation of every plant of a crop of twenty-four Acme tomato plants to another specific plant form bearing a new variety of fruit. The other relates to a subsequent exact duplication of that mutation in all its details as to both plant and fruit, in a crop of thirty plants, also of the Acme variety. It is apparent that both cases were initiated and consummated in the plants while they were growing in my garden because the germ cells which gave origin to the mutated plants were all formed there; and the mutated plants were there grown to maturity. Another fact, important in this connection, although stated in a previous paragraph, is that this new specific form had been previously produced by gardeners, who had given to it the name of potato-leafed tomato. That is, this one and the same species, L. solanopsis, has arisen suddenly and independently from L. esculentum at not less than three different times, each in a different locality.
Extraordinary as were the two cases of aggregate phylogenetic plant mutation which came under my observation, they are no more wonderful as natural phenomena than are the numerous cases of sudden and aggregate atavic reversion of previously constant and heritable fruit characters which have been mentioned in this and other publications. Indeed, among tomatoes, the aggregate occurrence of both plant mutation and atavic fruit reversion appears to be quite as normal as does their separate or individual occurrence. In both kinds of these cases, although their results are so different, the initial change has occurred in the germ cell of each of the seed ovules which gave origin to the affected plants. Both kinds are of mysterious, but doubtless natural, origin. Still, I can make no suggestion as to what may be the nature of either the determinate, predisposing or exciting cause in any of these cases.
It is not necessary, but it may not be inappropriate, to say that the foregoing paragraphs have not been written from a biometrical point of view, but from that of an old time naturalist. The principal facts which are there recorded have presented themselves to me with more force than I feel able to present them to others. I am still greatly impressed with their remarkable character, especially because they are not in accord with my own former views. Some of them also are known to be at variance with commonly accepted views of horticulturists, but I present them all with full confidence in their accuracy. Indeed, I do not admit the possible occurrence of any error that could have been instrumental in producing any of the phenomena which are here recorded.
Notwithstanding the peculiar features of these two cases of sudden mutation in the genus Lycopersicum, I assume that in their essential nature they are to be classed with those cases of mutation which have been observed in the genus Œnothera by Professor de Vries, and which he has used in demonstrating his theory of mutation. In his experiments with those plants, popularly called evening primroses, he repeatedly observed, in different years, the origination by sudden mutation of a few individual plants of one and the same species among the numerous progeny of the mother species. He also observed the similarly sudden and rare mutation of several new species from a mother species, but he has not reported any case of mutation of all the progeny of any one plant of a mother species; much less the progeny of a whole crop of plants, such as I have observed with reference to the genus Lycopersicum.
The number of specific mutations which were observed in Œnothera by Professor de Vries was greater than the number that has yet been observed in Lycopersicum; but the scope of mutative action in Œnothera embraced only a very small percentage of the abundant progeny of the mother plants; while in the two cases of mutation in Lycopersicum which I observed, that action embraced all the progeny of a small crop of mother plants. The mutative period in Œnothera occurred as a correlative of the extreme activity of natural reproductiveness and geographical distribution, but that period occurred in Lycopersicum as a correlative intensive cultivation. The unusual conditions, although so different in each case, apparently made the mutative opportunity available for the respective species. Other conditions will doubtless be found to give other species that opportunity, with diverse results. When other plants shall have been discovered in their mutative period the scope and diversity of mutative action will probably be found to differ in each case. If so, no one case can be made the absolute standard for such action.
The observations of Professor de Vries, as well as my own, show conclusively, not only that species may originate by sudden mutation, but that one and the same species may thus originate independently at different times and places and from different plants of a mother species. This fact is not without obvious significance in connection with geographical distribution of living species and the origination and distribution of organic forms during geological time.
- 'Varietal Mutation in the Tomato,' Science (n. s.), vol. xiv., pp. 841-844, New York, Nov. 29, 1901. 'The Saltatory Origin of Species,' Bull. Torrey Bot. Club, vol. xxix., pp. 511-522, New York, Aug., 1902. 'My Tomato Experiments,' The Independent, vol. liv., pp. 2460-2464, New York, Oct. 16, 1902. 'Aggregate Atavic Mutation of the Tomato,' Science (n. s.), vol. xvii., pp. 76-78, New York, Jan. 9, 1903.
- To avoid undue repetition, the terms 'species' and 'plant forms' are used interchangeably; and the term 'mutation' is used in its now accepted sense of sudden origination of species.
- For exhaustive discussions of this and kindred subjects see 'Intracellular Pangenesis' by Hugo de Vries. Jena, 1889; and also 'Die Mutationstheorie,' vol. 1, by the same author.
- These figures were originally published in an article by Dr. R. France, in Die Umschau, at Frankfurt am Main. In that article it was unfortunately stated that Fig. 1 represents the mother form, while the reverse is the fact.
- See 'Die Mutationstheorie,' von Hugo de Vries, Volumes 1 and 2, Leipzig, 1901, 1902; 'The Mutation Theory of Professor de Vries,' by Charles A. White, Smithsonian Report for 1901, pp. 631-640; and 'A New Theory of the Origin of Species,' by A. Dastre, Smithsonian Report for 1903, pp. 507-517.