Latitudinal variation in acorn size was examined in 32 species of the genus Quercus geographically restricted to eastern North America. Within-species comparisons showed the existence of a prevalent trend: individuals at high latitude produce, on the average, smaller acorns than individuals of the same species at low latitude. This observed trend, clines of decreasing acorn size with increasing latitude, could represent (i) the selected or plastic response to a shorter and (or) cooler growing season constraining the size of acorns that can mature on a tree, (ii) a consequence of past size-selective dispersal that accompanied postglacial range expansion, or (iii) a combination of these effects. The within-species trend contrasted with an observed among-species trend: species with large acorns have ranged that extend into higher latitudes than species with small acorns. Current evidence strongly suggests that small acorn size is maladaptive for successful seedling establishment at higher latitudes. Our results caution against inferring selective forces operating on seed size from among-species comparisons. Key words: acorn size, Quercus L., latitudinal clines, eastern North America.

Seed size is an important phenotypic trait commonly associated with the fitness of young seedlings exposed to environmental stress. Spring frosts can cause leaf necrosis and seedling mortality in young oak seedlings, and seedling mortality following spring freeze events is a factor limiting the northern range limits of Quercus gambelii (Nutt.). We examined the relationship between acorn size and seedling survival following spring freeze by exposing 1-month-old Quercus rubra (L.) seedlings from two sites in Massachusetts to −3 °C for 8 h. Mortality of 1-month-old frost-exposed seedlings (20–25%) was 10 times higher than control mortality, with survival directly related to acorn size. Seedling size at the end of the first growing season was negatively affected by frost exposure and positively associated with acorn size. Overwintering mortality (<5%) was negatively associated with seedling size. The production of large acorns should confer a selective advantage in habitats susceptible to spring frosts through a positive effect on seedling establishment success under stress. Keywords: red oak, acorn size, frost, seedling survival.

When fused with mouse L-cell cytoplasts, chick erythrocyte nuclei enlarge, take up proteins from the host cytoplasm, and recommence RNA synthesis. We found that during this transition the erythrocyte nuclei gain an internal nuclear matrix, thus providing a novel approach to questions concerning the nature of the salt-resistant intranuclear skeleton. A new method for preparation and examination of the nuclear matrix in situ is also described.The nuclear matrix is defined as the residual structure which remains after nuclei are sequentially subjected to DNase, low salt, and high salt treatments; additional extractions with nucleases and non-ionic detergents may also be included (reviewed in 4, 44). Despite the loss of ~90% of nuclear protein and up to 99% of nucleic acids, the final matrix retains the approximate shape and internal organization of the original nucleus. Three structural domains of matrix have been recognized: the peripheral pore complex-lamina, the residual nucleolus, and the interchromatinic or internal matrix (4, 44). The pore-lamina has been extensively characterized: its major components are a family of related structural proteins, the lamins (17,18,26,27,44,45), which seem to be confined to the nuclear perimeter (17,18,26). The internal matrix, however, contains a large number of polypeptides, most of which have yet to be studied (22,40,47).Current interest in the nuclear matrix has been stimulated by reports that the matrix is specifically associated with a number of key nuclear components, including newly synthesized DNA (6,7,9,14,23,38), newly synthesized RNA (2,16,20,21,24,33,34,36,47), steroid hormone receptors (2, 3), specific genes and families of repetitious DNA (13,37,39,42,46), and the polyoma T antigen (9). Recently, additional evidence has accumulated to implicate the matrix in RNA processing (12,20,35,43). On the basis of these results, it has been hypothesized that the internal nuclear matrix provides a skeletal framework in vivo upon and around which chromatin is organized and transcription and replication facilitated.We showed recently that inactive chick erythrocyte nuclei lack an internal nuclear matrix (29,31). The fact that these nuclei can be "reactivated" with respect to chromatin condensation and recovery of RNA and DNA synthesis if they are fused with another cell or cytoplast (1,15,19,28,32) suggested that it would be of interest to examine the status of the nuclear matrix during the reactivation process. We fend that within 16 h of fusion with mouse L-cell cytoplasts the chick erythrocyte nucleus gains an internal nuclear matrix. A preliminary account of this work has already been published (30). As discussed below, this system opens the way to definitive tests of various aspects of the nuclear matrix hypothesis. MATERIALS AND METHODSCell Cultures: Mouse L-929 ceils were maintained in minimal essential medium (Eagle) supplemented with 5% fetal calf serum, 100 U/ml pemcillin, and 100/Lg/ml streptomycin. For cell fusion experiments, cells were released from the...

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Abstract.One of the first symptoms expressed by declining trees is reduced growth in stem diameter and length increment. The possibility of a relationship between length increment and crown thinning in beech (Fagus sylvatica L.) was investigated by developing a computer model to simulate first order branching patterns of the apical 2 m of monopodially branching beech trees, 70-100 years old, for a range of length increment rates. The model was based on values for branching angle, main axis and branch length increment, number of branches produced per year and branch mortality rates for six healthy and declining trees. Shoot growth rates in the apical 2 m of the sample trees ranged from about 5 cm/year (decline class 3) to 43 cm/ year (healthy). Simulations of branching patterns in the apical 2 m of trees growing at different rates indicated that, when growth rate exceeded about 20 cm/year, total first order branch length and area explored were independent of growth rate. When growth rates fell below this value there was a reduction in total area explored and first order branch length due primarily to the formation of fewer branches. More acute branching angles contributed to a reduction in the area explored. Growth rate-related crown thinning could increase the risk of bark necrosis and secondary pathogen infection during dry and/or hot spells.