Chromosome numbers in the Myrtaceae.

THE MYRTACEAE have received comparatively little cytological attention, partly because of technical difficulties, and partly because the majority of species are relatively far removed from the centers of intensive study. Cytology in its more recent developments has proved its value in clarifving some of the problems of phylogeny, and it appeared likely that it might provide support for chemical, geographical and geological observations on the Myrtaceae. Neidenzu, in Die Naturlichen Pflanzenfamilien (1898), attributes 2750 species of tropical and warm temperate shrubs and trees to the Mvrtaceae. Andrews (1913) considered the family as consisting of approximately 3100 species. The family falls naturally into two subfamilies, the primarily fleshyfruited Myrtoideae with thirty-two genera of worldwide tropical distribution, and the capsular-fruited Leptospermoideae with forty genera. The best known species of Myrtoideae, Myrtus communis, is cultivated in Europe, although native to western Asia. With the exception of a monotypic genus in Chile, Tepualia, and extensions of Backhousia and Baeckea into Asia and Africa, the Leptospermoideae are largely limited to the Australasian region. Numerous ornamentals are found in both subfamilies. Eucalyptus, containing over 600 species and varieties (Blakely, 1934; 230 species, Willis, 1925), is an important source of fast-growing, durable woods and aromatic oils. Allspice is obtained from Pimenta; the guava from Psidium; and cloves from Eugenia. Studies of the Myrtaceae have been concentrated in Australia, and are, therefore, concerned mainly with the Leptosperinoideae. Difficulties imposed by intraspecific variation and interspecific hybridization are particulaarly apparent in taxonomic treatments of Eucalyptus (von Mueller, 1879-1884; Maiden, 1909-1929; Blakely, 1934). Analyses of the essential oils (Baker and Smith, 1902, 1920; Penfold and Morrison, 1927, 1932, 1934; McNair, 1942) have in the opinion of some workers established more reliable criteria for the delimitation of species, hybrids and varieties than strict morphological investigations. Summarizing the chemical analyses in their relation to the phylogeny of Eucalyptus, McNair states: "There has been orderly evolution in volatile oil characteristics of the Eucalypti which may be correlated with changes in morphological characteristics. However, oil groups represent cross-sections of the phylogenetic tree and not necessarily genetic sequences or natural groups of the systematist. Botanically distinct species are generally distinguished by their chemical constituents and also in some cases where. morphological examinations show little or no difference, chemical 1 Received for publication October 19, 1946. analysis of the oil reveals the existence of completely distinct varieties." Cytological contributions have dealt mainly witlh the Leptospermoideae. McAulay and Cruickshank (1937) described microspore development in-Eucalyptus. Smith-White (1942) studied meiosis in six genera-Angophora, Eucalyptus, Tristania, Backhousia, Leptospermum and Callistemon. Reports of chromosome numbers are found in various other papers (table 1). MATERIALS AND METHODS.-Chromosome counts were made for two genera-Psidium and Eucalyptus. Plants were obtained chiefly from the Division of Plant Introduction and Exploration, Department of Agriculture, and from Theodore Payne, Los Angeles, California, and were grown in a greenhouse at The Blandy Experimental Farm. Although Psidium produced fruit freely in the greenhouse, no Eucalyptus flowered. Herbarium specimens have been deposited in the Bailey Hortorium; specimen number is included in table 1. All counts were made from smears of young leaves stained with acetic orcein. Camera lucida drawings were made at a magnification of ca. 5000 and reduced in reproduction to ca. 3350. Previously reported chromosome numbers in the Myrtaceae as well as those from this study have been listed in table 1. General distribution of the various genera and the source of the plants are also included. The generic sequence followed is that of Neidenzu (1898); Blakely's Key to the Eucalypts (1934) was used in an attempt to reduce the synonymy among Eucalyptus collections although the species are listed in the table as they were named at the source. Species of Psidium are also listed as received; the only doubtful species is P. variabile which is probably a horticultural form of P. cattleianum. OBSERVATIONS.-Twenty-three Eucalyptus species were found to have a 2n number of 22. Two species, E. corynocalyx and E. redunca, are apparently aneuploids with 2n = 24; these are the only exceptions to 2n = 22 thus far reported among the Leptospermoideae. A similar constancy did not occur in Psidium: P. guajava has 2n = 22; P. cattleianum and P. cattleianum var. lucidum have 2n _88, which is to be considered an octoploid number. The chromosomes are small in both Psidium and Eucalyptus, ranging from approximately 1.22.5 ts. Although satellites and constrictions could occasionally be located, the leaf smear technique did not permit significant morphological study. DIscussIoN.-The Myrtaceae are, on the basis of the species thus far studied, an unlikely subject for intensive use of cytotaxonomic methods. Anderson (1937) has stressed the definite limitation on the use of chromosome number as a relationship criterion: "The taxonomic usefulness of information