AN EXPERIMENTAL APPROACH TO THE SYSTEMATICS OF THE MONOECIOUS‐CONJOINED MEMBERS OF THE GENUS CHARA, SERIES GYMNOBASALIA

Most monoecious-conjoined members of the series Gymnobasalia, genus Chara, bear either tetrasofitate or octoscutate antheridia. A determination of the extent to which various populations of these two morphologically distinct entities are reproductively isolated was the primary objective of the present study. Experimental crosses were attempted between emasculated sprigs and nonemasculated plants serving as sperm sources. All evidence suggests that tetraand octoscutate populations are reproductively isolated throughout their respective ranges even to the extent of producing black oospores. Crosses were also attempted between geographically isolated tetrascutate, and again octoscutate, populations. All were uniformly successful to the extent that black oospores resulted; but when such oospores were germinated many of the resulting progeny were found to be either nonviable or dwarfed, and frequently self-sterile as well. This was particularly true where parental clones differed with respect to geographical origin, chromosome number, or external morphological features. THE CHAROPHYTES have long been considered a taxonomically difficult group. At first glance there would seem little reason to doubt that they are fully deserving of this reputation for conflicting opinions continue to exist today as they have for the past century or more with respect to even the most common and widespread species. The source of such difficulties remains obscure. From time to time suggestions have been offered that the difficulties stem from: (A) widespread hybridization between species resulting in a plethora of intermediate forms, (B) complex interactions involving extreme phenotypic plasticity expressed throughout a variety of habitats, and (C) the occurrence of large numbers of morphologically similar, i.e. sibling, species. None of the contentions, however, are supported by adequate data; in the final analysis few would question that we simply do not know the source of such real or apparent difficulties. 'Until recently the relatively few students of the charophytes concerned themselves primarily with floristic surveys and for the most part alpha-level taxonomy. Few have undertaken experimental approaches, perhaps to some extent because of the misconception that charophytes either were difficult to culture or at best produced only abnormal growth. Over the past 15 years, how1 Received for publication 16 September 1970. Financial assistance was provided by National Science Foundation grant GB-8570. 2Present address: Department of Zoology, University of Wisconsin, Madison, 53706. ever, studies by MacDonald and Hotchkiss (1956), McCracken, Proctor, and Hotchkiss (1966), and Proctor et al. (1967) have demonstrated the feasibility of various experimental approaches including crosses between both emasculated monoecious plants and dioecious plants. At this point it should, therefore, be feasible to inquire into the systematic nature of certain widely distributed "species" with the objective of clarifying those taxonomic difficulties that have heretofore precluded the development of a stable system of classification. The present study is concerned with the taxonomic relationships of those monoecious-conjoined members of the series Gymnobasalia within the genus Chara (the "C. zeylanica complex"). Similar but more restricted studies of the C. braunii and C. globularis complexes have recently been completed (Proctor, 1970, 1971). That of the C. vulgaris complex is now in preparation. TAXONOMIC BACKGROUND-There exists within the genus Chara a sharply delimited group of plants known variously as series Gymnopodes Braun, series Gymnobasalia Zaneveld, or subsection Willdenowia Wood. Primarily as a matter of personal choice the term Gymnobasalia will be employed through the present study. All members the the series exhibit two unique and highly distinctive features: (A) the basal internodes of the branchlets are always ecorticate, and (B) recurved, depauperate-appearing branches, superficially reminiscent of the prop-roots of mangrove [The JOURNAL for October (58: 791-884) was issued 00 November 19711 AMERICAN JOURNAL OF BOTANY, Vol. 58: No. 10, 1971