Chaetospermum (M. Roem.) Swingle; Limonia subg. Chaetospermum M. Roem. (sec. Swingle and Reece 1967)
Swinglea Merrill (sec. Hodgson 1967, Mabberley 2004, Bayer et al. 2009; sensu Tanaka sec. Cottin 2002)
Crown compact or dense, not weeping. First-year twig surface glabrous; second- or third-year twig surface striate; thorns straight; prickles absent or not persistent. Petiole glabrous, length short or long; wings absent, if present, medium or wide, adjoining the blade or tucking beneath blade. Leaflets three, margin entire (by misinterpretation), crenate/crenulate or bluntly toothed, rachis wings absent, shade leaflet blades flat or weakly conduplicate, sun leaflet blades flat or weakly conduplicate. Leaflets not scented when crushed. Fruit as broad as long or longer than broad; rind variegated, medium green (4), light green with some break to yellow (5), or green-yellow (6); rind texture smooth (1-3), slightly rough (4-5), or medium rough (6-7); firmness leathery; navel absent.
Swingle and Reece (1967) provided the following additional notes on the genus:
"In 1913, Swingle published a description of this genus under the name Chaetospermum as follows: "A genus related to Belou [= Aegle], from which it differs in having persistent leaves with small rounded sessile lateral leaflets, fewer stamens (twice as many as the petals), fewer ovarial locules (8 to 10), an oblong ribbed fruit with a thick leathery rind and cells [locules] lined with a spongy tissue containing many large cavities or vacuoles.
'Leaves persistent, trifoliate, lateral leaflets small, sessile, usually less than one-third as long as the median, more or less blunt at the base or even rounded. Terminal leaflet gradually narrowed at the base. Petioles narrowly winged with a joint at the point of attachment of the leaflets. Spines slender, straight, sharp, in pairs at the axils or else one of the spines is replaced by a branch. Inflorescences axillary, composed of from one to several flowers on rather long, slender pedicels. Flowers perfect, 5-merous; calyx 5-lobed, petals 5, stamens 10, free. Pistil with a well developed style and a thick rounded stigma. Ovary with 8 to 10 cells [locules], each containing numerous ovules. Fruit oblong, longitudinally ribbed, with a very thick leathery rind, and with cells [locules] (filled with gum ?) surrounded with watery tissue containing large cavities or vacuoles. Seeds numerous in the long narrow cells, flattened ovate, hairy. Germination—cotyledons aerial, not increasing in size; first foliage leaves opposite, broadly ovate, subseriate, sessile, abruptly narrowed at base.' (Fig. 3-51.)
This genus differs rather widely from the other genera of the Hard-Shelled Citroid Fruit Trees (subtribe Balsamocitrinae) in having ellipsoid, or ovoid, ribbed fruits with a leathery outer shell showing very long, pointed, radially arranged oil glands in the peel and thick tissue containing mucilage glands lining the radial walls of the segments surrounding the seeds. Its paleate seeds resemble somewhat the woolly seeds of Aegle, and its trifoliolate leaves have a general similarity to those of Aegle, Balsamocitrus, and Afraegle, but its fruits differ widely in structure from those of other members of the subtribe.
Swinglea makes a good rootstock for Citrus if planted in soil that is always warm; it is a much better rootstock for Citrus than any other Hard-Shelled Citroid Fruit Tree that has been tested so far. Another indication of the rather close relationship of Swinglea to Citrus and the five other closely related genera constituting the True Citrus Fruit Trees is that it is the only genus outside of this group that is attacked in the wild state by citrus canker, a disease caused by the bacterial parasite Pseudomonas citri. Lee (1918, p. 664) stated concerning Swinglea glutinosa, which grows naturally and is also cultivated at the Lamao Experiment Station near Manila, P.I., not far from a large collection of Citrus species and cultivated varieties: "The susceptibility of C. [haetospermum = Swinglea] glutinosa to canker is easily greater than that of the sweet orange (Citrus sinensis) in the Philippines." It should be remarked that no natural infections were observed on any of the many other Citrus relatives under observation with respect to their susceptibility to citrus canker, although some of them showed slight susceptibly when inoculated by the needle-prick method (see Lee, 1918; Peltier and Frederich, 1920).
It must be remembered that the true taxonomic position of Swinglea is still somewhat in doubt and that it may prove to be rather closely related to the Primitive Citrus Fruit Trees, of which Hesperethusa and one species of Pleiospermium also show a certain degree of relationship to Citrus by being attacked, in the wild state, by a fungus belonging to the genus Elsinöe, closely related to E. fawcettii Jenkins that causes scab or verrucosis on several species of Citrus.
Burkillanthus, another of the Primitive Citrus Fruit Trees, of the subtribe Citrinae, shows striking similarities to Swinglea in its large fruits with stiff leathery rinds, its frequently trifoliolate leaves, and in the size, shape, and structure of its flower parts; but it differs widely from Swinglea in the anatomy of its fruits and in its seeds. It would be very desirable to make comparative studies of the development of the fruits of Burkillanthus and Swinglea and to make grafting tests in order to determine the degree of relationship of these two curious and little-known genera."
Bayer, R.J., D.J. Mabberley, C. Morton, C.H. Miller, I.K. Sharma, B.E. Pfeil, S. Rich, R. Hitchcock, and S. Sykes. 2009. A molecular phylogeny of the orange subfamily (Rutaceae: Aurantioideae) using nine cpDNA sequences. American Journal of Botany 96: 668–685.
Cottin, R. 2002. Citrus of the World: A citrus directory. Version 2.0. France: SRA INRA-CIRAD.
Hodgson, R.W. 1967. Horticultural varieties of Citrus. In: Reuther, W., H.J. Webber, and L.D. Batchelor (eds.). The Citrus industry, rev. University of California Press. http://lib.ucr.edu/agnic/webber/Vol1/Chapter4.html.
Lee, H.A. 1918. Further data on the susceptibility of rutaceous plants to citrus-canker. Journal of Agricultural Research 15: 661–665.
Mabberley, D. J. 2004. Citrus (Rutaceae): A review of recent advances in etymology, systematics and medical applications. Blumea 49: 481—498.
Peltier, G.L. and W.J. Frederich. 1920. Relative susceptibility to citrus canker of different species and hybrids of the genus Citrus, including the wild relatives. Journal of Agricultural Research 19: 339–362.
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Citrus ID Edition 2
October, 2011
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