. Wood; a manual of the natural history and industrial applications of the timbers of commerce. Wood; Timber. 14 OF WOOD IN GENEEAL be best understood from the diagrams (Fig. 9). In the thicken- ing of the cell-wall the area of the outer circle is at first un- thickened, but successive layers of thickening overlap this unthiekened area more and more so as to make a short canal broad at the end near the original cell-wall and narrow at the end towards the centre of the cell. Subsequently a shght thickening termed the torus forms m the centre of the unthiekened area. Pressure of hquid on one sid
. Wood; a manual of the natural history and industrial applications of the timbers of commerce. Wood; Timber. 14 OF WOOD IN GENEEAL be best understood from the diagrams (Fig. 9). In the thicken- ing of the cell-wall the area of the outer circle is at first un- thickened, but successive layers of thickening overlap this unthiekened area more and more so as to make a short canal broad at the end near the original cell-wall and narrow at the end towards the centre of the cell. Subsequently a shght thickening termed the torus forms m the centre of the unthiekened area. Pressure of hquid on one side of the pit-membrane often forces it against the " border," m which case the torus does not completely occupy the opening in the border or inner circle. The whole mechanism has been compared to a laboratory filter, the border being the funnel that acts as a support, the unthiekened mem- brane, which is permeable, corresponding to a filter-paper and the torus to the small platinum cone sometimes placed in the middle of the filter to protect it from direct pressure of liquid. The bordered pits on xylem vessels in Oak have been compared to. AJ AZ 0 FiGr 9 —Pits A, Simple pit; ^1, m tangential longitudinal section, A2,m surface view B, Bordered pit, Bl, m tangential longitudinal section , £2, the same, with the middle lamella thrust to one side; Bd, in surface view; J54, in semi-profile screw-heads, discs traversed by an elongated mark like the groove for a screw-driver, and the structure has been explained by the following imaginary model :^ " Imagine a pair of watch-glasses each pierced by a narrow sht, and imagine them united face to face with a dehcate circular piece of paper between them, and then fixed into a hole cut in a thick piece of card. The outhne of the screw- head is the outline of the umted watch-glasses where they are let into the card ; the groove in the screw-head is the oblique cleft which leads into the space between the ; In som
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Keywords: ., bookcentury1900, bookdecade1900, booksubjecttimb, booksubjectwood
