high variability of F 1 Citrus hybrids is, however, a very general phenomenon, not limited to a minority of the individuals. Further, enough is known, in general, of the production of new characters by new combinations of genes in crossing, to warn us against setting any narrow limits to the probable results of recombination in crosses between two highly heterozygous species. On the other hand, a new warning against undue confidence in gene stability is given by Eyster's (1924) recent hypothesis of qualitative division of certain genes. Possibly genes specially affecting other characters are sometimes unstable in the same way as are certain genes relating to variegation. If Citrus is especially notable for the occurrence of such unstable genes, this fact may account for part of the remarkable variability that is observed. POLYEMBRY•ONY Citrus seeds are frequently polyembryonic. Strasburger (1878, 1907) showed that the supernumerary embryos are formed by proliferation of nueellar cells surrounding the embryo sac. These adventitious embryos may be expected, therefore, to reproduce the seed-parent genotype, without variation due to segregation in sporogenesis or to recombination in fertilization. Citrus polyembryony is not entirely due to nucellar budding, however, for in 10 (probably 11) cases in our cultures, among more than 1000 hybrids, two hybrid seedlings have come from one seed. The seeds were planted separately, and all operations on which the reliability of the pedigrees depended were so carefully performed and checked that the single-seed origin of the pairs of seedlings is beyond doubt. The budding and consequent labeling were done with similar care. In eight of the ten cases, both of the original seedlings, as well as trees budded from them, have been positively classified as hybrids; in the ninth case, both of the small seedling trees are almost surely hybrids, as the budded trees certainly are; and in the tenth case one seedling died undescribed, so that its record depends on budded trees .alone. In the eleventh case, one of the two" seedlings" died young, and their separateness below the surface of the soil was not proved. Since in every case the two hybrids seem to be identical in type, in spite of the usual great diversity among hybrids of the same parentage, it is probable that these are all cases of "identical twins," each

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Citrus seeds often contain adventitious embryos, produced, usually subsequent to fertilization of the egg, by proliferation of nucellar cells adjoining the embryo sac.2 Seedlings developed from these embryos are therefore entirely "maternal" in their inheritance, and in interspecific crosses they can usually be readily distinguished, even in advance of fruiting, from the hybrids that result from fecundation. Among the Citrus progeny from cross-pollination that are now growing in the breeding orchard at the Citrus Experiment Station, the proportion of "apogamic" individuals usually ranges about from 20 to 85 per cent of the whole number of seedlings of a seed-parent variety. Besides the ordinary apogamic progeny, another form that shows no characters of the pollen parent, and therefore is obviously produced asexually, has frequently been found.

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For more than a month subsequent to the birth of her dilute young, the mutant was so feeble that all hope was given up for her recovery. After this, however, she rallied sufficiently to produce a second litter of three. These all developed intense pigmentation, proving the mutation to have been germinal and to have involved probably one only of the two gametes from which she was derived.

INTRODUCTIOKUMEROUS genetic variations affecting portions of Citrus trees have N occurred under cultivation. "Bud-variation" or "bud-mutation" forms have been studied extensively from the horticultural viewpoint in various commercial varieties, such as the Eureka and Lisbon lemons (SHAMEL et al. 1936), but little is known about the causation of the changes concerned.Certain conspicuously mixed bud-variation forms, such as lemon "strains'' with white-margined leaves (SHAMEL 1932) and orange strains with two kinds of fruit rind (SHAMEL et al. 1925)' appear to be essentially periclinal chimeras which include the parent type as one component. In other cases, as with the horticulturally important Wase satsuma (TANAKA 1932), it seems very doubtful whether frequent somatic reversion of a bud-variant form to the parent type is due primarily to an originally chimera1 condition or to genetic instability of the new type.The extreme diversity of Citrus forms and the very abundant heterozygosis which seems to be general in this genus (WHITE 1914, FROST 1926 suggest that gene mutation occurs comparatively often. This process may therefore be considered as one probable cause of bud variation in Citrus, although the conditions are unfavorable for its discrimination from somatic crossing over or structural aberrations in chromosomes."Twin" variations in rind characters, which change adjacent sectors in opposite directions from the parental condition, are occasionally observed. These presumably indicate the occurrence of abnormal mitosis. When, as occasionally happens, both rind thickness and rind color are changed in opposite directions in two adjacent se.ctors, some shift involving more than one gene locus seems clearly indicated. Aside from such cases (and germlayer shifts in chimeras, of course), the general nature of bud variation in Citrus seems likely to be determinable only when a cytologically visible alteration is involved.Of the possible kinds of cytological evidence on the causation of bud Paper No 464, University

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