A Chromosome Count and Range Extension for Christensenia (Marattiaceae)

Abstract

Of the six genera recognised by Copeland (1947, pp. 14-17) in the Marattiaceae, there are cytological records in the literature for Angiopteris, Marattia and Danaea. These numbers are: n = 40 (Manton & Sledge, 1954; Mehra & Singh, 1955) and n = 80 (Manton & Sledge, 1954; Ninan, 1956) forAngiopteris; n = 39 (Brownlie, 1961), n = 78 (Ninan, 1956), and n = 40 (Walker, in Manton, 1959; Walker, 1966) for Marattia; and n = 80 and triploid hybrids showing 40 bivalents and 40 univalents for Danaea (Walker, 1966). These chromosome counts show thatAngiopteris and Danaea have a base number of 40, whereas Marattia has two base numbers, 39 and 40, the lower number being derived by the loss of one chromosome (Walker, 1966). The cytological uniformity of the representatives which have been examined so far underlines the distinctive morphological, anatomical and embryological characteristics shared by the genera (Campbell, 1911; Bower, 1926). No cytological information is available for two of the remaining genera, Archangiopteris, Macroglossum, or, until now, for Christensenia. Christensenia is unique in the Marattiaceae in possessing palmately divided fronds with scattered, round synangia and reticulate venation. It is found in Assam, Malaysia, Sumatra, Java, Bali, Borneo, and the Philippines. Throughout its range several species have been described, but these are doubtfully distinct and, pending critical taxonomic investigation, all the variants are perhaps best regarded as forms of one species, C. aesculifolia (Blume) Maxon. The genus has not been reported previously from the Solomon Islands, but in the 1960's it was found by different collectors on three of the major islands. During the 1965 Royal Society Expedition to the British Solomon Islands, C. aesculifolia was found by the author on San Cristobal, the southeasternmost island, growing in the lowland forest approximately seven miles inland from Wainoni Bay near the confluence of the Warahito and Pagato Rivers. It was evidently very localized, since considerable searching in the area revealed only two small populations in deep shade on the steep banks of two small calcareous streams feeding the Pagato and Sumaro Rivers respectively (RSS 4215, 28 July 1965, K; RSS 4220, 29 July 1965, K). In addition, specimens are known from the Buin area, Bougainville (25 July 1964, Schodde 3674, CANB) and from Allardyce Harbour, Santa Ysabel (4 Feb 1967, BSIP 8271, HON, K). These records represent a notable extension of the range of the genus eastwards into Melanesia and place Christensenia in a small group of Malesian fern genera now known to reach their southern limit in the Solomon Islands. It is of some phytogeographical interest to note that, so far as the author is aware, the genus has not yet been found in New Guinea, although this may merely be a reflection of insufficient botanical exploration.