We have analysed the surface antigen phenotype of a human embryonic stem (hES) cell line (H7) and the changes that occur upon differentiation induced by retinoic acid, hexamethylene bisacetamide and dimethylsulphoxide.The undifferentiated stem cells expressed Stage Specific Embryonic Antigen-3 (SSEA3), SSEA4, TRA-1-60, and TRA-1-8 but not SSEA1. In these characteristics they closely resemble human embryonal carcinoma (EC) cells derived from testicular teratocarcinomas, and are distinct from murine EC and ES cells. The undifferentiated cells also expressed the liver/bone/kidney isozyme of alkaline phosphatase detected by antibody TRA-2-54, the class 1 major histocompatability antigens, HLA-ABC, and the human Thy1 antigen. Differentiation of hES cells was induced by retinoic acid, HMBA and DMSO with the appearance of various cell types including neurons and muscle cells. The surface antigens characteristically expressed by hES cells were down-regulated following induction of differentiation and other antigens appeared, notably several ganglioside glycolipids detected by antibodies VIN-IS-56 (GD3 and GD2), VIN-2PB-22 (GD2), A2B5 (GT3) and ME311 (9-O-acetyl-GD3). Whereas the expression of HLA was slightly down-regulated upon differentiation, its expression was strongly induced by interferon-γ in both the undifferentiated and the differentiated cells, although the induction in the differentiated cultures was considerably stronger than in the stem cells. In all of these features the human ES cells, and their pattern of differentiation, resembled the pluripotent human EC cell line NTERA-2 although clearly the range of cells generated by the hES cells was considerably greater.

SUMMARYFailure of regeneration of Tilia cor data in north-west England is associated with almost complete sterility of the seeds. Between 1964 and 1979 significant numbers of fertile seeds were recorded only after the exceptionally warm summer of 1976. In contrast, large or moderate crops of fertile seed were produced at sites in central and southern England in at least 8 of these years.Comparative studies of pollination, fertilization and seed development at sites in north-west England, central England and northern Erance show relatively small differences in the proportions of fiowers which are pollinated but significant differences in the rates of extension of pollen-tubes. In samples collected in 1977 from north-west England germinated pollen was present on most receptive stigmas but many pollen-tubes extended only a short distance down the style and the few which reached the base appeared to be arrested in the ovary wall. In French samples pollen had germinated on all receptive stigmas and numerous pollen-tubes had reached the ovary witbin 2 days of pollination.Tbese differences cannot be attributed to variation in tbe incidence of self-pollination, as at least some trees of T. cor data in nortbern populations are almost certainly self-fertile. Tbe differences are correlated witb temperatures at tbe time wben tbe samples were collected. Wben pollen-tubes are growing in sucrose solutions, tbeir rate of extension shows a large response to temperature over tbe same range (15 to 25 °C). Analysis of records sbows tbat temperatures in nortb-west England at tbe time of fiowering are frequently too low to permit fertilization.Tbis is confirmed by studies of tbe development of ovules. Altbougb a bigb proportion of ovules in T. cordata lack embryo sacs, normally at least one is functional in eacb ovary. In over 90% of fruit formed in nortb-west England, embryos are contained in none of tbe 10 ovules. In less tban 10% of fruit partly developed embryos are present, suggesting tbat temperatures are also too low in late summer to allow completion of tbeir development.

No abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Ecology.

SUMMARY Seedlings and 2‐year old saplings of Tilia cordata growing in the shade on sandy‐textured soils can survive periods when the water potential of the soil decreases to −18 to −55 bars. Their mycorrhiza formed by Cenococcum geophilum is shown to remain alive. Measurement of rates of water‐loss from seedlings in similar conditions decreases from 16.0 to 26.2 mg h−1 when the soil is wet, to 4.1 to 4.6 mg h−1 when the soil is dry. Calculation of the volume of the mycorrhiza shows that its water content would support transpiration for periods not exceeding 1 h. Survival of the mycorrhiza through long periods of drought is apparently related to the ability of the fungal partner to withstand desiccation.