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Anatomy of the endocarp was studied in relation to the physical dormancy-breaking mechanisms in experimentally treated Rhus aromatica var. aromatica and R. glabra germination units, which include seed plus endocarp (hereafter seeds). The endocarp has three distinct layers, with brachysclereids on the outside, osteosclereids in the middle, and macrosclereids on the inside. Brachysclereids in the carpellary micropyle region (i.e., region immediately adjacent to the integumentary micropyle) are shorter than those in other parts of the endocarp, and the macrosclereids in this region are not elongated. Thus, a weak point is formed in the endocarp. Concentrated sulfuric acid broke seed dormancy in R. aromatica by eroding the brachysclereids and osteosclereids in the carpellary micropyle region, whereas boiling water broke dormancy in seeds of R. glabra by inducing a blister adjacent to the carpellary micropyle.

Summary 1 The clonal perennial herb Helianthus divaricatus and the clonal shrub Rhus aromatica reach the northern limit of their distribution in southern Quebec (Canada), where both are rare, but they are more common southwards in the adjacent province of Ontario. We tested whether peripheral populations of these species maintain themselves by having highly variable individual growth and survival rates. Their annual rates of population growth, λ, would then vary more, from year to year, than those of more southern populations in neighbouring Ontario, where the species are common. 2 Demographic analyses compared, over two transition intervals, size-based transition matrices for two northern (Gatineau Park, southern Quebec) and two southern populations (Pinery Park, southern Ontario) of each species. Comparisons used log-linear analyses, bias-corrected λ-values with 95% confidence intervals, and elasticity values. 3 For both species, the northern populations showed a larger variation between transition intervals and local populations in some of their vital rates and in λ-values. For H. divaricatus, elasticity values combined with log-linear analyses indicated that differences in the growth and survival of medium-sized ramets contributed most to the differences in λ-values between northern and southern populations. For R. aromatica, they indicated that differences in the fate of both medium and large ramets contributed to variation in λ. 4 Most of our data indicated a greater demographic variability in the northern populations of H. divaricatus and R. aromatica. At the northern periphery, these species grow in very few of the numerous habitat patches that appear to be suitable for them, whereas the two species together present a near continuous cover in all suitable habitats further south. High demographic variability therefore appears to contribute to determining the distribution of plant species, by increasing the extinction probabilities of local populations at the distribution limit.

Morphology and physiology of fruit and seed development were compared in Rhus aromatica and R. glabra (Anacardiaceae), both of which produce drupes with water-impermeable endocarps. Phenology of flowering/fruiting of the two species at the study site was separated by ;2 mo. However, they were similar in the timetable and pattern of fruit and seed development; it took ; 2m o and;1.5 mo for flowers of Rhus aromatica and R. glabra, respectively, to develop into mature drupes. The single sigmoidal growth curve for increase in fruit size and in dry mass of these two species differs from the double-sigmoidal one described for typical commercial drupes such as peach and plum. Order of attainment of maximum size was fruit and endocarp (same time), seed coat, and embryo. By the time fruits turned red, the embryo had reached full size and become germinable; moisture content of seed plus endocarp had decreased to ;40%. The endocarp was the last fruit component to reach physiological maturity, which coincided with development of its impermeability and a seed plus endocarp moisture content of ,10%. At this time, ;50, 37, and 13% of the dry mass of the drupe was allocated to the exocarp plus mesocarp unit, endocarp, and seed, respectively. The time course of fruit and seed development in these two species is much faster than that reported for other Anacardiaceae, including Rhus lancea, Protorhus,and Pistacia.

Premise of the study: Microsatellite markers were developed for Spondias radlkoferi to assess the impact of primate seed dispersal on the genetic diversity and structure of this important tree species of Anacardiaceae. Methods and Results: Fourteen polymorphic loci were isolated from S. radlkoferi through 454 GS-FLX Titanium pyrosequencing of genomic DNA. The number of alleles ranged from three to 12. The observed and expected heterozygosities ranged from 0.382 to 1.00 and from 0.353 to 0.733, respectively. The amplification was also successful in S. mombin and two genera of Anacardiaceae: Rhus aromatica and Toxicodendron radicans. Conclusions: These microsatellite loci will be useful to assess the genetic diversity and population structure of S. radlkoferi and related species, and will allow us to investigate the effects of seed dispersal by spider monkeys (Ateles geoffroyi) on the genetic structure and diversity of S. radlkoferi populations in a fragmented rainforest.

Important in the bioenergetics of granivores is the difference in energy content of seeds from plants grown on different areas within the organism's normal environment. To determine possible differences in energy content, seeds of nine species of plants (Solanum rostratum, Helianthus annuus, Rhus aromatica, R. glabra, Ambrosia trifida, Cornus drummondi, Symphoricarpos orbiculatus, Setaria lutescens, and Bromus japonicus) were collected from four distinct range sites. No differences were detected (except for Rhus aromatic) between mean energy content of seeds from the four sites. However, significant intraspecific differences (P < 0.05) were noted between means of energy content of seeds from different locations within the same range site.