Chimaera

CHIMAERA. This term is used in botany to apply to cer tain types of plants formerly known as "graft-hybrids." The term graft-hybrid suggests a plant which is a true hybrid pro duced by grafting, i.e., a plant produced by the fusion of cells de rived respectively from the two plants grafted together. Such a union of cells would be of the nature of fertilization (see CY TOLOGY) and the offspring, having the characteristics of the two plants grafted, i.e., the stock and scion, would be of composite origin and so might properly be called a graft-hybrid. The de velopment of a hybrid plant in such a way is, however, entirely unknown. It is true that plants composite in nature and having some of the characteristics of the two plants employed may de velop from the graft, but they arise in an entirely different way without fusion of cells. The term graft-hybrid which used to be applied to them has therefore been dropped and they are now called "plant-chimaeras" or simply "chimaeras." A chimaera was a mythological monster of composite nature, having the head of a lion, the body of a goat and the tail of a dragon. The plant chimaeras are truly of composite nature and origin.

These chimaeras, though not under this name, have been known in gardens for some time, but it is only comparatively re cently that their nature has been understood. In 1825 a French horticulturist grafted a shoot of Cytisus purpureus (the ordinary English broom, Cytisus scoparius, is an allied species) on the trunk of the ordinary laburnum, Laburnum vulgare. M. Adam was very surprised to find that a shoot developing at the graft was interme diate in character between the two parents or, rather, showed some characters of C. purpureus and others of L. vulgare.

The original composite plant has been propagated vegetatively and is fairly common in gardens, being usually called Cytisus Adami or Laburnum Adami of ter its producer. It was supposed to be a "graft hybrid" as were also the so-called Crataego mespilus forms which arose at Bronveaux in Lorraine as a result of a graft between two different genera Crataegus monogyna and Mespilus germanica. In both cases it was observed that certain shoots on the composite plant would revert or throw back to the original forms, i.e., to L. vulgare or C. purpureus in the one case or to C. monogyna or M. germanica in the other. Another supposed case of a graft hybrid was that of the Bizzaria orange which it was presumed had arisen from a graft between Citrus aurantium (the ordi nary orange) and C. media.

The Discovery of the Nature of "Graft Hybrids."—In 1907 our knowl edge of this subject passed into a new phase. In that year H. Winkler published in Germany some experimental work on grafting Solanum nigrum (a common Brit ish plant with black berries sometimes known as black nightshade) as scion on Solanum Lycopersicum (the tomato) as stock. After the graft had taken, a trans verse cut was made through it at the junction of the stems. From the surface of contact of the stock and scion a num ber of buds developed which grew out into new shoots. Most of these were just shoots of tomato or of nightshade, but one of them was composite in nature, having the characters of tomato on one side and of nightshade on the other. Some of the leaves which arose at the junction of the two halves had partly the char acters of one species and partly of the other. Winkler very appro priately termed this shoot a chimaera since it was half of one species and half of another. In 1908 Winkler published the re sults of an examination of 268 grafts, between the same two spe cies, which bore more than 3,00o shoots when cut across in the way described. Of these, five were chimaeras of the kind de scribed the year before, but one was intermediate between the stock and scion, being rather nearer to the nightshade than the tomato. This shoot Winkler believed to be a real "graft-hybrid" to which he gave the name Solanum tubingense since it was pro duced at Tubingen. Next year he claimed to have produced sev eral more graft-hybrids to which he gave special names, S. Darwin ianum, S. Gaertnerianum, S. proteus and S. Koelreuter. It was found, however, that seedling plants (arising of course sexually from these forms) always reverted to the nearer parent, never producing hybrid seedlings. Again if the new forms were crossed with the nearer parent-form the product was always pure tomato or pure nightshade. This is not the behaviour which one would expect from a true hybrid.

Work on Variegated Plants.

In 1909 E. Baur published ob servations on variegated plants which shed a new light on the nature of the so-called graft-hybrids. It is well known that the garden geranium (really Pelargonium zonale) often shows leaves with a white margin. When examined microscopically it is found that there are two or three layers of colourless cells cover ing the internal green tissue, and these tissues are separated right up to the growing point so that the variegated Pelargonium plant body consists of a green core surrounded with a white sheath. To such a plant Baur gave the name periclinal chimaera, i.e., a chimaera in which one component invests the other. He further suggested that Winkler's so-called graft-hybrids between tomato and nightshade were also periclinal chimaeras, i.e., plants with a tomato sheath over a core of nightshade tissue or a sheath of nightshade over a core of tomato tissue. This would explain why these forms produce seedlings of either tomato or nightshade, since the sexual cells arise solely from the central tissue, the outer cells not being concerned. Baur also examined Cytisus Adami and the Crataego-mespilus "hybrids" and found that they also were peri clinal chimaeras. Cytisus Adami has a core or body of Laburnum vulgare with a skin (epidermis) of Cytisus purpureus. In the other case the chimaeras have either a Crataegus core with a Mespilus skin (epidermis) or vice versa. Winkler on examining his so-called Solanum "hybrids" in the light of Baur's suggestion found that Baur was correct in his surmise. Solanum tubingense has a nightshade core and a tomato skin one layer thick. S. proteus has also a nightshade core and a tomato skin but in this case the skin is thicker, having two layers.

S. Koelreuterianum has a tomato core and a nightshade skin one layer thick.

Classes of Chimaeras.

Winkler produced his chimaeras ex perimentally by "cleft grafting" of main shoots, a tongue of the scion being inserted in a cleft in the stock. When the two shoots had united the graft was cut across transversely. The relation of parts would then be as shown in fig. za where the black area represents the scion and the light area the stock. The cut surface of the graft produces "callus" and from this callus buds develop. Most of these are "pure" in origin since they arise from callus tissue which has developed from either stock or scion ; but some may arise from callus, which is partly of stock and partly of scion origin, and these may give chimaeras. The tissues in the shoots arising from the buds of mixed origin may be arranged as in fig. 1b, which is a periclinal chimaera, or as in fig. 1c, which is a sectorial chimaera, only a section of the tissue in cross-section (the black portion) being of scion nature. The first chimaera Winkler produced was of this type, though in that case the chi maera was a "half and half" one. The third type shown in fig. id was formerly considered as sectorial, but it is really an in complete periclinal for which the term mericlinal has been sug gested (see bibliography under Jorgensen and Crane) . True sec torial chimaeras are very rare, the plant first obtained by Winkler being of this type, as are also a few Pelargoniums. The periclinal type is quite common, e.g., Cytisus Adami, the forms of Crataego mespilus, Pelargonium, Bouvardia. The chimaera may be a whole plant or a shoot only or only a smaller portion such as a flower or fruit. In plants such as Bouvardia, which is of the complete periclinal type, we have the interesting result that shoot cuttings and root cuttings produce plants of different types ; in one case the new plant arises from one set of tissues, in the other case from the other set. It has recently been shown that chimaeras are more common than was suspected. The tubers of certain varieties of the common potato are shown to be chimaeras. If the shoots developing from the normal eyes are rubbed off, the new one developing arises from deeper tissues and so from the other component of the chimaera, thus giving shoots of a different type.

literature of this subject is largely in German. H. Winkler, "Ueber Propfbastarde and pflanzliche Chimaren," Berichte der deutsche botanische Gesellschaft (19o7) ; ibid. (5908) ; "Ueber das Wesen der Propfbastarde, Periklinalchimaren, and Hyperchimaren," ibid. (1910) . There is a good review of the literature up to 1911 in the Botanical Gazette (191I) . For later papers in English, see W. Bateson, "Root Cuttings and Chimaeras," Journal of Genetics (Cambridge, 1916 and 1921) ; C. A. Jorgensen and M. B. Crane, "Formation and Morphology of Solanum Chimaeras," ibid. (1927).

(V. H. B.)