TAXON 35(2): 243-246. MAY 1986
ON THE IDENTITY OF ISOETES TRIQUETRA A. BRAUN
R. James Hickey•
Summary
Vegetative characters of Isoetes are generally thought to be oflittle diagnostic value because of their plasticity and limited number of distinctive character states. The neglect of these vegetative features has resulted in incomplete species descriptions and taxonomic confusion. This problem is exemplified by I. triquetra, a name based on a single sterile collection, which has previously been assumed, but never shown, to be synonymous with I. andina. A comparative study ofligule morphology indicates that the two names are indeed synonymous.
The taxonomy of Isoetes is based to a great extent on the form and distribution of megaspore surface morphology features (ornamentation). While recent studies on the genus (Boom, 1982; Hickey, 1981; Kott and Britton, 1983; Taylor et al., 1975) support the view that such spore features represent the most reliable source of taxonomic information, this dependence on a single suite of characters results in a number of problems. The most obvious is the difficulty in identifying sterile collections. More importantly, however, is that the dependence on megaspore features has been at the expense of adequate and complete characterization and depiction of vegetative character states. In many cases, vegetative characters are considered oflimited taxonomic value because of their infraspecific variability or their interspecific uniformity. It has been well documented that many vegetative characters vary as the result of phenotypic plasticity, ontogenetic development and age of the plant. However, ignoring apparently variable characters is not an acceptable substitute for documentation of their variability. Most systematic studies on Isoetes have been provincial in scope and have been primarily concerned with closely related species which show few significant differences in their vegetative features. As a result, characters showing little or no systematic variation for a given region have been neglected despite the fact that they may be of systematic importance to the genus as a whole. Most frequently this problem (one which is compounded by space limitations in many journals) takes the form of abbreviated, incomplete species descriptions which do not include apparently obvious and assumed data. Examples of poorly described character conditions in Isoetes are phyllotaxy and root branching. These two character conditions were considered to be oflittle systematic value because spiral phyllotaxy and dichotomously branched roots were assumed to be universal throughout the genus. This assumption changed with the publication of/. tegetiformans Rury (Rury, 1978), a remarkable species which has a distichous phyllotaxy and unbranched roots. The discovery of/. tegetiformans indicates the need for complete data bases either in the form of species descriptions or discussions of character variability (e.g., Matthews and Murdy, 1969). Another situation supporting the need for better species characterization pertains to the identity of/. triquetra A. Braun. In British Ferns, Hooker (1861) submerged all previously published species of Isoetes into synonymy under the European/. lacustris. In the same article he recognized as distinct a species from Tasmania and another species from Quito, Ecuador. The Ecuadorian species was accompanied by a list of distinguishing characteristics and Spruce's manuscript name, "Isoetes Andina." Isoetes andina is a robust, amphibious 1
Botany Department, Miami University, Oxford, OH 45056, U.S.A.
MAY 1986
243
Table I. Comparison of Isoetes andina and "1. saracochensis."
Megaspores
Microspores
Ligule
I. andina
"I. saracochensis"
echinate to densely cristate 550-970 ~tm X= 770 ~tm
rugulate 400-620 ~tm
echinate to echinate-verrucate 40-55 ~tm long x = 47 ~tm
echinate to verrucate 30-36 ~tm long X= 33 ~tm
triangular, strongly cordate; cushion triangular to ovate, hastate
widely deltate to widely ovate, weakly cordate; cushion depressed ovate, not hastate
x=515~tm
to nearly terrestrial species ranging from western Venezuela to south central Ecuador. In 1862, A. Braun published the name I. triquetra for what appeared to be the same or at least a very similar species. Braun, impressed with the similarities between the two plants, stated "Spruces I. Andina von den Anden Quito's, welche zu sehen ich noch nicht Gelegenheit hatte, mag nach dem, was Hooker (British Ferns l.c.) dariiber angiebt, eine ahnliche oder vielleicht dieselbe Art sein." Based on the similarities of the two descriptions and the overall morphological similarity of the type collections, nearly all subsequent authors have considered Braun's I. triquetra to be synonymous with I. andina. Unfortunately, Hooker's earlier and valid publication of I. andina has been largely ignored or the name has been placed in synonymy under I. triquetra (e.g., Kubitzki and Borchert, 1964; Pfeiffer, 1922; Rauh and Falk, 1959; Weber, 1922). The conspecific treatment of I. andina and I. triquetra is somewhat surprising because the type collection of I. triquetra was made in southern Peru some 1400 kilometers from the nearest known station of I. andina in southern Ecuador. The assumed conspecificity is even more amazing considering the well known variation exhibited by vegetative characters, their supposed unreliability and the fact that Braun's type is sterile, and hence lacks both megaspores and microspores for technical verification. More recent collections of Isoetes, made within 200 km of the type locality of I. triquetra in southern Peru, further complicate the situation because of their strong vegetative resemblance to the types of both I. andina and I. triquetra. This species, "1. saracochensis" Fuchs nom. nud. (Fuchs-Eckert, 1982), is readily differentiated from I. andina on the basis of megaspore, microspore and ligule characters (Table 1). The megaspores of I. andina are echinate to densely cristate and range in size from 550-970 ~m with a mean diameter of 770 ~m. Megaspores of "1. saracochensis" are distinctly rugulate and range from 400-620 ~m with a mean diameter of 515 ~m. The microspores of the two species are similar in surface morphology, having a basically echinate exospore pattern, but differ considerably in size; those of "1. saracochensis" range from 30-36 ~m long (x = 33 ~m) while those of I. andina are 40-55 ~m long (X.= 47 ~m). The ligules of the two species are also diagnostic, those of"I. saracochensis" are deltate to widely ovate and weakly cordate with a depressed ovate cushion, while those of I. andina are triangular and strongly cordate with a triangular to ovate, hastate cushion. The presence of an Isoetes in southern Peru which closely approximates I. triquetra (and consequently I. andina) in such vegetative characters as a firm, rigid and triquetrous subula (the distal, non-laminate portion of the leaf), a thick cuticle, broad and extensive alae (the proximal laminate portions of the leaf), and an acute leaf apex presents a further complication by suggesting an alternative identity for I. triquetra. The presence of"I. saracochensis" presents complications, but it also provides a potential solution. In this case, the ligule, a well differentiated, vegetative organ on the adaxial leaf surface, can be used not only to 244
TAXON VOLUME 35
~1 A
8
c
0
Fig. 1. Leaf bases and ligules of Isoiites species. A) /. andina Megasporophyll base showing rudimentary velum coverage and a triangular ligule with its elongate, hastate cushion (ffllgaard & Balslev 10145, AAU). B) I. triquetra tropophyll base with its triangular ligule and elongate, weakly hastate cushion (Lechler 3337, B). C) "1. saracochensis" megasporophyll showing nearly complete velum coverage and a deltate ligule with its depressed ovate cushion (Chavez 2323, GH). Ligular cushion stippled.
discriminate it from the northern I. andina but is also useful in providing evidence regarding the identity of I. triquetra. A comparison of the ligules of"I. saracochensis" with those of I. andina and I. triquetra (Fig. 1) leave little doubt as to the identity of the latter. The ligules on the type of I. triquetra, even though partially eroded, are distinctly similar to those of I. andina. Both are long triangular and both show a hastate base to the thickened, central cushion. Such a hastate base is not evident in the cushion of "I. saracochensis" either in specimens with complete ligules or in those where the ligule margins have eroded and only the central cushion persists. Evidence from ligule morphology supports the previously assumed synonymy of I. andina and I. triquetra. The case for the taxonomic synonymy of these two names is strengthened by the fact that no other species of the Central Andes has a ligule morphology similar to that of I. andina (Hickey, 1985). The synonymy of the common, northern Andean, paramo quillwort which has gone under the names I. andina and I. triquetra is as follows: Isoetes andina Spruce ex Hook., British Ferns 1861. Type Collection: Paramo de Naba, Quitinian Andes, Oct 1858, Spruce s.n. (Holotype: K. Isotypes: B!, UPS-fragment!. Photos ofB specimen: S!, UPS!). I. triquetra A. Braun, Verh. Bot. Vereins Prov. Brandenburg 3(4): 332-333. 1862. Calamaria triquetra (A. Braun) Kuntze, Rev. Gen. Pl. 2: 828. 1891-1893. /. lechleri var. triquetra (A. Braun) L. D. Gomez, Brenesia 18: 5. 1980. Type Collection: in pascuis humidis, Sachapata (am ostlichen Abhang der Cordillera von Peru), Lechler 3337 (Holotype: B!. Photos of B specimen: S!, UPS!). /. triquetra var. dissimile Rodriguez, Bol. Mus. Cienc. Nat., Caracas 1: 59. 1955. Type Collection: Venezuela, Edo. Merida, laguna de Mucubaji, 3500 m, 21 Dec 1955, Rodriguez 316 (Holotype: YEN!).
The classification of any group of plants on the basis of a single character or on a single suite of character states presents inherent difficulties and is self-supporting. Even in cases where particular characters appear too variable or too stable to be of systematic value within a localized region or when circumventing a particular group their documentation is important because these characters may prove useful in more inclusive studies. The situation is particularly true for Isoetes where vegetative characters rarely show significant variation at the local level but are useful phylogenetic markers on a more global scale. MAY 1986
245
Literature Cited Boom, B. M. 1982. Synopsis of Isoetes in the southeastern United States. Castanea 4 7: 38-59. Braun, A. 1862. Anhang iiber einige ausllindische Arten der Gattung Isoetes. Verh. Bot. Vereins Prov. Brandenburg 3(4): 326-333. Fuchs-Eckert, H. P. 1982. Zur heutigen Kenntnis von Vorkommen und Verbreitung der siidamerikanischen Isoetes-Arten. Proc. Ned. Akad. Wetensch. C85: 205-260. Hickey, R. J. 1981. A new Isoetes from Jamaica. Amer. Fern J. 71: 69-74. - - . 1985. Revisionary studies of Neotropical Isoetes. Ph.D. Dissertation, The University of Connecticut. Hooker, W. J. 1861. Britishferns. London. Kott, L. and D. M. Britton. 1983. Spore morphology and taxonomy of Isoetes in northeastern North America. Canad. J. Bot. 61: 3140-3163. Kubitzki, K. and R. Borchert. 1964. Morphologische Studien an Isoetes triquetra A. Braun und Bemerkungen iiber das Verhaltnis der Gattung Stylites E. Amstutz zur Gattung Isoetes L. Ber. Deutsch Ges. 77: 227-234. Matthews, J. F. and W. H. Murdy. 1969. A study of Isoetes common to the granite outcrops ofthe southeastern Piedmont, United States. Bot. Gaz. 130: 53--61. Pfeiffer, N. E. 1922. Monograph of the Isoetaceae. Ann. Missouri Bot. Gard. 9: 79-232. Rauh, W. and H. Falk. 1959. Stylites E. Amstutz, eine neue Isoetaceae aus den Hochanden Perus. I. Teil: Morphologie, Anatomie, und Entwicklungsgeschichte der Vegetationsorgane. Sitzungsber. Heidelberger Akad. Wiss. 1959: 1-83. Rury, P. M. 1978. A new and unique, mat-forming Merlin's-grass {lsoetes) from Georgia. Amer. Fern J. 68: 99-108. Taylor, W. C., R. H. Mohlenbrock and J. A. Murphy. 1975. The spores and taxonomy of Isoetes butleri and /. melanopoda. Amer. Fern J. 65: 33-38. Weber, U. 1922. Zur Anatomie und Systematik der Gattung Isoetes L. Nova Hedwigia 63: 219-262.
246
TAXON
VOLUME 35