Scanning electron microscopy has been used to examine silica isolated by chemical means from the wood of 32 species of woody perennial. The silica consists of aggregate grains lying free in the lumina or in ray and xylem parenchyma cells in 24 of the species. It occurs as dense silica in the other species, filling the lumina or lining the internal surfaces of vessels (and fibres) in all cases except Gynotroches axillaris where it is deposited in ray parenchyma cells. Infrared spectra and X-ray diffraction diagrams, obtained for specimens of both sorts of silica, are indistinguishable from those for amorphous silica. Aggregate grain and dense silicas are also alike in that their differential thermal analysis curves show a rather broad endothermic peak between 175° and 205°C. The results are discussed in relation to possible modes of deposition of the two sorts of silica and the tendency for silica in ray parenchyma cells to be associated with polyphenols.
The mechanism by which the stems of trees recover after being bent or tilted out of the vertical is considered. The mechanical consequences of the tendency for the main stem apex to reerect itself and grow vertically or to be replaced by a vertically growing lateral branch, and of the asymmetrical radial growth of displaced stems, are pointed out. The asymmetry usually develops on the upper side of a bent or tilted hardwood stem and on the under side of a softwood stem. Since such asymmetry can usually be accounted for on the basis of accentuated development of wood rather than bark, the question of the possible functional significance of the wood arises. Experiments are cited to demonstrate that the wood (reaction wood) is effective as a means for assisting displaced stems to recover, contracting longitudinally on the upper side of a hardwood stem and expanding on the under side of a softwood stem. Consideration is then given to the structure, differentiation, and chemical composition of the reaction wood formed by hardwoods and softwoods with a view to locating the force active in aiding recovery and determining its origin. The consequences of associating the support of a still-growing stem displaced from the vertical with the idiosyncracies of secondary wall formation of reaction wood cells, and the reerection of such a stem with lignification of these walls, are explored. On this basis, the hypothesis is advanced that the recovery force is located in the region of differentiating reaction wood cells undergoing lignification, the active force arising from the swelling of cell walls as a result of deposition in them of lignin. Objections to the hypothesis are mentioned.
The surface architecture and location of megakaryocytes in the extravascular compartment of mouse bone marrow are considered. The finger-like cytoplasmic processes that extend into sinuses pass through apertures, approximately 3.0 millimicron in diameter, occurring singly or in groups. The cytoplasmic processes enlarge on first entering a sinus and form villi that seem to anchor them to endothelium. The villi consist largely of microfilaments. The cytoplasmic processes extend, attenuate, and undergo irregular constriction along their lengths. The considerations demarcate modicums of cytoplasm of platelet size one from another that are released when they break. Extravascular platelet release may also occur, and evidence is presented that points to the likelihood of the platelet's being engulfed and phagocytosed by macrophages before reaching the microcirculation.
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