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The Genus Diellia and the Value of Characters in Determining Fern Affinities 1

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The Genus Diellia and the Value of Characters in Determining Fern Affinities 1 THE GENUS DIELLIA AND THE VALUE OF CHARACTERS IN DETERMINING FERN AFFINITIES 1 Warren H. Wagner, Jr. IN THE CURRENT renaissance of fern taxonomy it occasion to suggest the relative value of certain would seem important to emphasize the nature and characters in the taxonomy of ferns, and to use as relative value of the characters which can be used as a specific example the genu3 Diellia of Hawaii. This evidence of relationship. Certain allegedly "con­ genus seems to be an ideal choice for a symposium servative" characters, especially features of the sori, on fern classification because it has been placed at the characteristic aggregations of sporangia-such one time or other in association with no less than as their position in relation to the veins and leaf four different assemblages of ferns-groups which margins, their outlines, and protective indusia­ are now interpreted as belonging to tour separate have frequently been used as absolute criteria of families. Such authors as Moore (1857), Hille­ affinities in the higher leptosporangiate ferns. An­ brand (18881, Christensen (1925), and Bower other character which has frequently been em­ (1928) regarded Diellia as being a relative of Lind­ ployed is leaf form-the outline of the blades and saea, the latter genus now construed to be a mem­ leafie.s, the venation pattern, and the degree of dis­ ber of the Pteridaceae. Hooker and Baker (1883) section. These are conspicuous charac.ers, easily and others associated Diellia with Dictyoxiphium, seen with the naked eye or under a hand lens. But, which Copeland (1947) has recently shown to he­ unfortunately for the superficial observer, the ferns long to the Aspidiaceae. Arbelaez (1928), Holttum show numerous and striking undoubted examples (1949), and others placed it with such genera as of parallel or convergent evolution in regard to Davallia and Nephrolepis, both of the Davalliaceae. these gross features. In the case of leaf form, the In 1929 Copeland adduced evidence that Diellia is well-known "reniform ferns," for ins.ance, all have actually related to the family Aspleniaceae. But in leaves which are nearly alike in appearance; but all except the last interpretation of its affinities, the these are entirely unrelated ferns, as illustrated by sorus of Diellia was the primary basis of cornpari­ such species as Adiantum renilorme, Pterozoniuni son. In this genus the sorus is located in the mar­ reniforme, Cardiomanes renijorme, and Lindsaea ginal position and the indusium opens outward to reniformis. The sorus, in spite of the fact that much the pinna margin, just as it does in Lindsaea and of our classification has been Iounded on it, is itself the genera related to that genus, as well as in Da­ equally liable to parallel or convergent evolutionary vallia and its immediate relatives, and also in cer­ changes. Indeed, at one time practically all ferns tain species of Nephrolepis. According to Bower. which posse5S round sori but no indusia were placed in the phylogeny of those ferns which he consid­ in "Polypodium"; and ferns in which the sporangia ered to be related to Diellia, the sorus occupied just covered the dorsal pinna surfaces were put in an such a marginal position originally; and along vari­ all-inclusive genus, "Aerostichum," but now it is ous lines of advancing evolution, the sorus was evident in case after case that this was an artificial considered to have "slid" to the dorsal laminar sur­ and unnatural disposition. Even so recently as 1928, face. Bower divided the ferns into a "marginal Bower considered the familiar genus Athyrium as series" and a "superficial series," and after a study being closely related to Asplenium because these of preserved material from Hawaii, he placed Diel­ two genera possess approximately the same soral lia among the marginal ferns, along with the lind­ type. However, information available now on such saeoid ferns and the davallioid ferns (1928). A varied characters as scale structure, stipe anatomy, fern genus which has been interpreted so variously sporangial features, and cytology in the respective as Diellia represents an obviously difficult taxonomic genera, rather firmly establishes the Iact that the problem, but similar situations are repeated in nu­ gross soral resemblance is probably only a coinci­ merous other genera, and these greatly need re­ dence, an example of convergent evolution. investigation. The disagreement which centered The purpose of this paper is to discuss and on around the interpretation of Diellia could onlv be 1 Received for publication September .3, 1952. resolved by an exhaustive examination of all its This manuscript was prepared at Harvard University characters that could be studied, and by looking at during the tenure of a Gray Herbarium Fellowship, and is the genus from the broadest possible standpoint. It based on a paper given in a joint Symposium of the Arner­ is now evident from the study of Diellia and from ican Society of Plant Taxonomists and the American Fern Society on the "Classification of Ferns" held at the 1950 comparing it with other cases that we do not ac­ meetings of the A.T.B.S. in Columbus, Ohio. Most of the tually know a priori what is "conservative" and factual information used here is drawn from investigations taxonomically dependable as evidence in any par­ made in the Department of Botany, University of Califor­ ticular line of evolution. It seems necessary and nia, Berkeley, and reported in a monograph of the genus obvious, therefore, to adopt a procedure involving Diellia (Wagner, 1952). It is a pleasure to express thanks for help and suggestions to Professors Lincoln Constance, a total comparison of all the available characters of E. B. Copeland. A. S. Foster, and Reed C. Rollins. ferns if we are accurately to determine their rei a- January, 1953] WAGNER-GENUS DIELLIA AND PHYLOGENETIC CHARACTERS 35 tionships. As Dr. Copeland has indicated, we are side of the exospore shallowly concave and the distal required to study and compare the whole plant. more strongly convex. The exospore itself is col­ When this viewpoint is adopted it becomes appar­ orless, smooth and hyaline, but it is surrounded by ent that many characters have been overlooked or a rugose, brown-pigmented perispore and has dark have not been exploited by workers of the past. The "perisporial spines." The presence of a perispore following discussion may serve to illustrate how by itself stands in sharp contrast to the ferns of valuable some characters can be as sources of the lindsaeoid affinity, for in the latter a perispore evidence. is entirely absent. In addition the lindsaeoid spores To the extent that they may be analyzed, the are tetrahedral in form. Such davallioid examples habitats of ferns may, in some instances, contribute as Davallia and N ephrolepis possess the elliptic at least an inkling, however small, to their complex spore outline as in Diellia, but the perispore is also comparative physiology. And whether it be taxo­ absent here, and the exposed exospore wall surface nomically revealing or not, their ecology should be is sculptured. Among the asplenioid ferns, and es­ investigated as a possible source of information. pecially in certain of the rock-spleenworts, the Some of our lack of information on habitat is ac­ spores almost exactly match those of Diellia in counted for by the lamentable practice of making structural details and pigmentation. collections entirely without ecological data. In the The study of the morphology of the fern gameto­ present problem, fortunately, there is a considerable phyte in laboratory cultures has recently been stim­ amount of information at hand. Diellia is a genus ulated by the new phylogenetic and taxonomic inves­ comprising species which occur always in dry, tigations of ferns. There is still too limited shaded, rocky situations, in wooded but arid knowledge of this generation to suffice for its broad gulches, and which apparently can tolerate but little evaluation as a character in taxonomy, although competition from other ground-story ferns and flow­ there is little question that contributions have been ering plants (Wagner, 1950). On the other hand, and will be made from studies of this generation, as Lindsaea and its relatives are ferns primarily of has already been indicated by the researches of the wet forest and damp stream banks. Such species Stokey (1951). The peculiarities of these simple, or relatives of Lindsaea as do occur in dry places usually haploid, plants which seem especially to of­ tend to be "sun-ferns," tolerant of more or less di­ fer sources of comparative evidence are the follow­ rect insolation (for instance, Lindsaea ensifolia and ing: (1) mode of germination, (2) cell plate for­ Sphenomeris chusanai . Davallia and related genera mation, (3) structure of gametangia, and (4) pres­ are obligate or facultative epiphytes, a condition ence and nature of the haploid trichomes. But to entirely unknown in any species of Diellia. While adopt the view of Orth (1936), that the system of the large and diverse genus Asplenium in the trop­ fern classification should be replaced by a "natural ics has a broad amplitude of ecological situations one" based on the gametophyte, is not warranted among its various species-groups, there is one of its and is inconsistent with the ideal of comprehensive groups, that of the "rock-spleenworts" so frequent total comparison as the basis for taxonomy. In the in temperate climates, that has habitat requirements study of Diellia there is at present a severe restric­ as a whole rather conspicuously similar to those of tion in the utilization of this character because of Diellia. Evidence drawn from the ecological pref­ the absence or paucity of studies of the gameto­ erence of these ferns could in this case be corre­ phytes of the other genera involved.
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