Monographs Details: Cassia
Authority: Isley, Duane. 1975. Leguminosae of the United States: II. Subfamily Caesalpinioideae. Mem. New York Bot. Gard. 25 (2): 1-228.
Description:Genus Description - Trees, shrubs, herbs. Leaves 1-even-pinnate; leafstalk often with one or several characteristic glands. Stipules various, deciduous or persistent. Flowers in axillary or terminal, simple or congested and compound racemes, or appearing solitary to fascicled (Chamaecrista) because of raceme reduction. Calyx of nearly separate, subequal or unequal, imbricate to valvate sepals; petals equal to unequal, usually yellow; functional stamens 10 or fewer, often of several sizes and forms, the lower typically largest, the upper three frequently reduced and sterile; anthers typically basifixed and dehiscent by an apical or basal pore or slit. Legume diverse, dehiscent or indehiscent, usually elongate, compressed to terete, often large, valves commonly coriaceous to ligneous; endocarp often segmented between transversely positioned seeds, sometimes also with a longitudinal septum. Seeds few-numerous.
Chamaefistula G. Don
Chamaesenna (DC.) Raf. ex Pittier
Pseudocassia Britt. & Rose
Psilorhegma (Benth.) Britt. & Rose
Pterocassia Britt. & Rose
Tharpia Britt. & Rose
Xerocassia Britt. & Rose
CBN x = 7,8.
Primarily tropics and warm regions of all land masses, perhaps 600 species. Ours including both native and numerous introduced kinds, the native mostly herbaceous, of s extremities of the country except for 4 that extend n in the e and c temperate United States, the introduced primarily trees and shrubs for ornamental planting in s Florida, urban California and s Texas.
Cassia is one of the three or four largest genera of legumes and Irwin and Turner (1960) have considered it among the 25 largest of dicotyledons. Bentham’s exemplary revision (1871) established an infrageneric classification, summarized and reevaluated by Irwin (1964), yet largely viable. As understood by these workers, Cassia consists of three subgenera, Fistula (now properly subgenus Cassia), Senna and Lasiorhegma sharply defined by characters derived from the androecium, pod, and seeds, the subgenera Senna and Lasiorhegma further resolving naturally but less exactly into eight or nine sections. For a genus of this size and diversity, efforts to detach one or more of its constituent parts have been minimal, the work of Britton and Rose (1930) on species of the New World constituting a lonely exception. In modern times, the only generic segregate from Cassia native to the United States that has found serious support is Chamaecrista, originally described by Moench (1794), revived by Greene (1897), and adopted by Britton and Rose (1930) who listed 111 North American species. Chamaecrista has been more recently supported by Senn (1938b) on the basis of cytological evidence and followed by Isely (1958). Amato-Avanzi (1956) and Pantulu (1960) have provided cytological reinterpretation. These workers felt that a distinctive morphological aspect combined with basic chromosome number (x = 8) believed to be different from genuine Cassia seemed a sound basis for treating Chamaecrista as distinct. But Irwin and Turner (1960) and Irwin (1964) on the basis of broader information from tropical American species have demonstrated this premise to be based on too limited a cytological and morphological sample. The enduring soundness of Bentham’s judgment seems validated.
Among dozens of papers reporting chromosome numbers in Cassia, those of Senn (1938a), and Irwin and Turner (1960) deal with sufficient numbers to allow phylogenetic speculation, and Jacob’s (1940) classic treatise relates interesting genome considerations. The preponderance of Cassia possess a base chromosome number of 7. A base of n = 8 in subsection Leiocalyx is presumed to be a consequence of aneuploid gain (Irwin and Turner, 1960). Chromosome numbers in multiples of 6 occur sporadically through most subgenera and sections of Cassia. Among several hypotheses, Irwin and Turner (1960) consider incidental aneuploid loss most likely. Most species of subgenus Lasiorhegma are diploid; the remainder are tetraploid or seldom octoploid, the only exceptions being C. auriculata and C. alata reported as both diploid and tetraploid (Irwin and Turner, 1960; Pantulu, 1960; and earlier papers cited by these authors). Among many determinations, Irwin and Turner (1960) encountered no meiotic irregularities such as univalents, multivalents, rings.
It is possible that some reports of polyploidy based on root-tip determinations are a consequence of seedling somatic polyploidy, a phenomenon of not uncommon occurrence in the Caesalpinioideae and Mimosoideae, but rare in the Papilionioideae (Berger et al., 1958; Isely, 1970).
Several features useful in both phylogenetic assessments of and identification of Cassia have been omitted or passed over lightly in this treatment for specific reasons:
(1) Androecia. Variation in number, form, dehiscence, and pubescence of the stamens is of proven systematic value in Cassia, but careful dissection may be required for observation of refined detail. Since other, more easily grasped differential characters are generally available, it has seemed superfluous to dwell on the androecia, beyond noting the general conformation of anthers and the number of functional ones found in each flower.
(2) Pods. Form, texture, and internal septation of the pods, together with the attitude of the seeds relative to the pod’s axis and superficial characters of the seeds’ testa are evidently of major phylogenetic significance in the genus, but have low pragmatic value as aids to identification of specimens. Mature fruits are seldom available; those of some groups are massive, or fleshy, are stored with difficulty, and can be examined effectively only after destructive sectioning. For this reason the pods are described only in general terms and the seeds ignored except for number.
In descriptions of Cassia fasciculata and its relatives in the distinctive subsection Chamaecrista, I refer to axillary bracteate fascicles, although the inflorescence is in reality a condensed axillary or supra-axillary raceme. For convenience also in descriptions, the peduncle-pedicel axis of these flowers is termed the pedicel.
Flower color in most Cassia is usually yellow, with red marks at the base of 4 petals in most U.S. section Chamaecrista, but color changes as flowers age to golden-yellow, orange or orange-red. This results in a variety of color terms used in descriptions and on herbarium tickets.
Many woody cassias are delightful or even spectacular flowering trees and shrubs, and the number introduced, especially to Florida, is legion (Menninger, 1958, 1959). Few of these exotic species are represented in U.S. herbaria, either by cultivated or native material, in such quantity as to permit critical taxonomic evaluation. Herein lies an unavoidable limitation in this account. In difficulties I have received helpful advice from Howard Irwin.