Decalcification of echinoderm tissues for electron microscopy can be achieved after glutaraldehyde-osmium fixation by treatment with a 1:1 mixture of 2% ascorbic acid and 0.3 M NaCl for 12-24 hours. Electron photomicrographs of material decalcified by this procedure are superior to those from EDTA-treated tissue and show few of the deleteriour effects produced by EDTA.

41explain this. At higher face velocities the perrneability of the loaded filter is lower which Ieads to a higher pressure build-up inside the bag during the cleaning pulse. Klingel [28] found that up to 80% of the total emission for a given filtration period can originate during these first 30 seconds. At higher face velocities particulate emissions increases even further than the values set out in fig. 1.28, because cleaning frequency must be increased to maintain apresetdifferential pressure. These investigations show how important the cleaning system is to the separation behaviour; it emphasises also the importance of selecting fllter materials and cleaning conditions to achieve optimum results. As cleaning intensity increases more cleaning air is required, and the clean gas dust concentration increases. Careful study is required to select filtering speeds that will keep emission Ievels within acceptable Iimits. The answer to these questions must be deterrnined specifically for each particular set of circumstances (dust behaviour, filter medium, cleaning mode). Our present state ofknowledge does not provide universal solutions; more research is required.

The dormant (mid‐November to mid‐February) microsporangia of Pseudotsuga menziesii (Douglas‐fir) contain pollen mother cells (PMC's) in diffuse diplotene, surrounded by 1–2 layers of tapetal cells and 3–4 layers of microsporangial wall cells. At the beginning of dormancy, PMC's are large and their walls are lysed. The cell walls contain a thick layer of loosely‐arranged fibrils which are produced in large vesicles in the PMC cytoplasm and are secreted across the plasma membrane. PMC's contain several layers of rough ER. The inner tangential and the radial walls of the tapetal cells are lysed. During dormancy the PMC's form many new autophagic vacuoles, the chromatin consists of a network of fine threads comprised of medium‐sized granules of uniform size and the nucleoli split. The outer tapetal wall is thick and becomes encrusted by an irregular lipid layer. The tapetal cytoplasm is similar to the PMC cytoplasm but is devoid of amyloplasts. The tapetal cytoplasm shows secretory activity at the beginning of dormancy and again near the end of dormancy. The later secretory activity results in the deposition of a spongy material, especially along the radial and inner walls of the tapetal cells. Tapetal cells contain 1–2 large nuclei which show prominent and irregular clumps of chromatin. Subcellular developmental changes occur in the dormant microsporangia of Pseudotsuga in much the same manner as has been reported for Pinus.

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. This content downloaded from 204.235.148.92 on Wed, 30 Dec 2015 16:57:16 UTC All use subject to JSTOR Terms and ConditionsAmer. J. Bot. 70(9): ABSTRACT The dormant (mid-November to mid-February) microsporangia of Pseudotsuga menziesEi (Douglas-fir) contain pollen mother cells (PMC's) in diffuse diplotene, surrounded by 1-2 layers of tapetal cells and 3-4 layers of microsporangial wall cells. At the beginning of dormancy, PMC's are large and their walls are lysed. The cell walls contain a thick layer of loosely-arranged fibrils which are produced in large vesicles in the PMC cytoplasm and are secreted across the plasma membrane. PMC's contain several layers of rough ER. The inner tangential and the radial walls ofthe tapetal cells are lysed. During dormancy the PMC's form many new autophagic vacuoles, the chromatin consists of a network of fine threads comprised of medium-sized granules of uniform size and the nucleoli split. The outer tapetal wall is thick and becomes encrusted by an irregular lipid layer. The tapetal cytoplasm is similar to the PMC cytoplasm but is devoid of amyloplasts. The tapetal cytoplasm shows secretory activity at the beginning of dormancy and again near the end of dormancy. The later secretory activity results in the deposition of a spongy material, especially along the radial and inner walls of the tapetal cells. Tapetal cells contain 1-2 large nuclei which show prominent and irregular clumps of chromatin. Subcellular developmental changes occur in the dormant microsporangia of Pseudotsuga in much the same manner as has been reported for Pinus.AMONG THE CONIFERS which have been studied, microsporangiate strobili are initiated the summer before they shed pollen but the stage of development at which they overwinter may vary. Some genera (e.g., Pinus) overwinter soon after sporangenous cells form; other genera (e.g., Abies, Picea) overwinter soon after pollen mother cells (PMC's) form or after PMC's have entered meiotic prophase (e.g., Larix, Pseudotsuga, Thuja, Tsuga) while certain species of other genera (e.g., Chamaecyparis, Juniperus) overwinter after microspores have formed (Owens, 1980). The acquisition of the overwintering, or dormant, state in woody perennial tissue is a gradual process marked by many developmental and physiological changes (Pomeroy and Siminovitch,1971; Li and Sakai, 1978). The structure of the overwintering tissues, especially reproductive tissues, of conifers is poorly understood.Although the ultrastructure of the microsporangia has been investigated in Pinus during various stages of microsporogenesis and pollen development (Willemse,

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