A New and Unique, Mat-Forming Merlin's-Grass (Isoetes) from Georgia

Abstract

The quillworts, Isoetes (Lycopsida, Isoetales), constitute a heterosporous family of about 60 species recognized by botanists since the time of Linnaeus as having fleshy, two-, three- or four-lobed corms with a crown of spirally arranged, ligulate microphylls and numerous dichotomizing roots that emerge in series along the median furrow(s) of the corm. Vertical stem growth in Isoetes results from a shoot apical meristem that produces sterile and fertile leaves and a central core of primary vascular tissue. Expansion growth is effected by the activity of a lateral meristem or cambium that encloses the primary vascular cylinder. During growth in girth, the original corm structure is maintained by radial displacement and seasonal loss of the outer, older leaves, roots, and corm tissue. Sexual reproduction involves the formation of micro- and megasporangia, the spores of which develop endosporically into male and female gametophytes. Water-mediated cross fertilization produces daughter sporophytes. Developmentally, young sporophytes exhibit a distinct change in their overall morphology (Paolillo, 1963). During the first 10-15 plastochrons, leaves are produced alternately on either side of the embryonic shoot apex. During this stage, the apical meristem produces neither cauline vascular tissue nor primary stem growth; thus the vasculature of these distichous juveniles consists of a sympodium of traces to leaves and roots on either side of the shoot apex and median furrow. The resultant two-ranked leaf arrangement is lost, however, as soon as the apical meristem becomes large enough to initiate leaf primordia in a spiral sequence. Apparently, this apical enlargement also is responsible for the differentiation of the vascular core and the concomitant inception of vertical stem growth. Vegetative propagation is rare in Isoites. Goebel (1879) reported it in I. echinospora Dur. Goebel (1905, p. 431) also reported adventitious plantlets on I. lacustris leaves in a sporangial or sub-sporangial position in sterile specimens from the Vosges Mountains of France. Similar sterile but viviparous plants of I. lacustris were reported from Windmere, England by Manton (1950, p. 255). The developmental origin of such epiphyllous plantlets is not understood and is in need of investigation. Branching in the upper portions of Isoetes axes, another mechanism of vegetative reproduction, also has been reported (Motelay & Vendryes, 1882; SolmsLaubach, 1902; Eames, 1936, p. 51), and a recent developmental study has revealed basal branching in two species of Isoetes (Karfalt & Eggert, 1977). With the exception of branching and bud-formation, the quillworts traditionally have been considered as a morphologically uniform family of pteridophytes. The discovery of the Peruvian genus Stylites (Amstutz, 1957), however, has expanded