SUPERNUMERARY CHROMOSOMES IN SAXIFRAGA VIRGINIENSIS (SAXIFRAGACEAE)

Acetocarmine squashes of root tips have demonstrated that 2n = 20 and 38 in Saxifraga virginiensis. These contrast with the earlier reported count of 2n = 28 for this species. In several populations supernumerary chromosomes were detected. Both intrapopulational and interpopulational variation in supernumerary chromosome number were detected, with the largest number of supernumerary chromosomes observed being six. Because these supernumerary chromosomes are equal in size to many of the smaller A chromosomes during mitotic metaphase, the presence of supernumerary chromosomes in this species could not be ascertained by analysis of mitotic metaphase preparations alone. During mitotic prophase, however, the supernumerary chromosomes of S. virginiensis are highly heterochromatic, appearing more densely coiled and darkly stained than the A chromosomes. This characteristic facilitated the recognition of supernumerary chromosomes in this species. The similarity in size of A and supernumerary chromosomes during mitotic metaphase and the observation of six supernumerary chromosomes in one population suggest that the count of 2n = 28 reported earlier for S. virginiensis may actually be a misinterpretation of 2n = 20 plus 8 supernumerary chromosomes. Furthermore, these findings and the observation of this same supernumerary chromosome phenomenon in other species of Saxifraga raise the possibility that some of the many disparate chromosome counts attributed to aneuploidy in the large genus Saxifraga may also be the result of misinterpretations of supernumerary chromosomes as A chromosomes.     

ULTRASTRUCTURE OF TETRASPOROGENESIS IN THE MARINE RED ALGA PTILOTA HYPNOIDES1

Both tetrasporangia and dormant apical cells of short vegetative filaments of the marine red alga Ptilota hypnoides have been examined by electron microscopy. Various cytoplasmic inclusions readily distinguish the vegetative apical cells from the reproductive apical cells which become tetrasporangial mother cells. The transformation of tetrasporangial mother cells into mature tetrasporangia involves a series of cytoplasmic changes which can be correlated with specific changes in the investing wall layers. The extracellular changes provide the basic criteria for the division of tetrasporogenesis into 3 successive stages. The ultrastructure of each stage is described and discussed in relation to the current knowledge of red algal cytology. In addition, a possible mechanism for the liberation of spores and gametes of red algae is proposed.     

DEVELOPMENT OF THE OVULE AND MEGAGAMETOPHYTE IN SAXIFRAGA HIERACIFOLIA

Wiggins , Ira L. (Stanford U., Stanford, Calif.) Development of the ovule and megagametophyte in Saxifraga hieracifolia. Amer. Jour. Bot. 46(10): 692–697. Illus. 1059.—Buds of Saxifraga hieracifolia collected in the vicinity of Point Barrow, Alaska, fixed, sectioned, and stained by standard methods, revealed that the archesporial cell in the ovule of this species is hypodermal and gives rise to the megaspore mother cell and a small number of parietal cells. Occasionally 2 megaspore mother cells occur within an ovule. Meiosis in the megaspore mother cell produces a linear tetrad of megaspores, the chalazal one of which normally gives rise to a monosporic, Polygonum-type megagametophyte. The polar nuclei fuse near the chalazal end of the megagametophyte and the antipodal cells disintegrate prior to fertilization. A distinct filiform apparatus and a marked lateral “spur” develop on each synergid. Vacuolation in the egg cell and in the synergids follows the usual pattern. Only a single integument surrounds the nucellus.     

AN EXPERIMENTAL TEST FOR REPRODUCTIVE INTERACTIONS BETWEEN TWO SEQUENTIALLY BLOOMING SAXIFRAGA SPECIES (SAXIFRAGACEAE)

The blooming period of Saxifraga reflexa on south-facing bluffs in interior Alaska precedes and slightly overlaps that of Saxifraga tricuspidata. We tested the hypothesis that reproductive interference maintains this phenological separation by measuring seed set of potted 5. reflexa during the normal and 5. tricuspidata blooming periods, with and without 5. tricuspidata nearby. During the S. reflexa blooming period we placed arrays of four pots of S. reflexa in sites with and without 5. tricuspidata. Several weeks later, during the S. tricuspidata blooming period, we repeated the experiment with potted 5. reflexa whose flowering had been artificially delayed. Seed set of 5. reflexa was decreased by retarding blooming time, but not by the presence of blooming S. tricuspidata. Although both species were visited by the same pollinators, primarily syrphid flies, the presence of S. tricuspidata did not reduce the rate of visitation to S. reflexa. In mixed plots, visitation rates by syrphid flies were much higher to 5. reflexa than to S. tricuspidata. Interspecific floral transitions by syrphid flies were uncommon. It appears that in 1986 at this site S. reflexa experienced no negative effects from flowering synchronously with, and occurring in close proximity to, 5. tricuspidata.     

STRUCTURALLY PRESERVED LEAVES OF HARRIMANELLA HYPNOIDES (ERICACEAE): PALEOECOLOGY OF A NEW NORTH AMERICAN LATE PLEISTOCENE FOSSIL

Plant macrofossils from the basal inorganic sediments at Tom Swamp, north central Massachusetts, contained leaves, seeds, and fruits of arctic-alpine species, nearly all of which occur at present near the summits of Mt. Washington and Mt. Katahdin (New England) and in other alpine and arctic areas northward in eastern North America. Included were the needle-leaves of the dwarf shrub, Harrimanella hypnoides (L.) Coville, which matched comparative modern leaf samples in all anatomical details. The macrofossil assemblage was deposited before 12,830 ± 120 radiocarbon years ago and prior to the expansion of spruce populations in the region. Fossils of H. hypnoides suggest that snow beds were a regular feature of the summer landscape of southern New England during late glacial time. Calculations using the average lapse rate indicate that the mean annual paleotemperature in the Tom Swamp area may have been depressed 8–9 C below present means.     

EMBRYOLOGICAL STUDIES IN THE FAMILY SAXIFRAGACEAE (S.L.). I. DEVELOPMENT OF THE OVULE AND EMBRYO SAC IN SAXIFRAGA FORTUNEI VAR. PARTITA (MAKINO) NAKAI

The development of the ovule, megaspore and megagametophyte in Saxifraga fortunei var. partita (Makino) Nakai was observed. The ovule is anatropous, bitegmic, and crassinucellate. Both integuments originate from the epidermis. The archesporium is considered to be multicellular. The primary sporogenous cell functions as the megaspore mother cell which forms a T-shaped tetrad. The chalazal member of the megaspore tetrad is functional and develops into a Polygonum-type embryo sac. In the pyriform synergids the filiform apparatus is observed, but any hook or indentations could not be recognized. The antipodal cells are detectable until the Helobial endosperm undergoes several nuclear divisions. Secondary multiplication of the nuclei or the cells of the antipodals could not be observed.     

Brachyptera Group. Sp.32. C. Brachyptera. Sp.33. C.c RUFA. Sp.34. C. Troglodytes. Sp.35. C. Fulvicapilla.

A part from considerations as to where it is best wedged into the linear sequence, the brachyptera group is defined in general terms as a compact group of four small or very small species which resemble one another in many important specific characters and the other thirty-six species classified here as Cisticola in so many ways of form, coloration and behaviour as to make them best understood by classifying them also under that generic name.