. A manual of botany. Botany. 290 MANUAL OF BOTANY cells or fibres which form part of the soft portions of the vascular bundles of the higher plants [fig. 626). It generally happens that when cell-walls are being thick- ened, the new deposits of cellulose are not laid down uniformly over the surface, but instead form ridges or prominences of various patterns, with unthickened portions between them. The thickening may take the form of a spiral band, winding obliquely round the interior of the cell; it may be deposited in the form of rings placed regularly along its greater axis ; or it may be i
. A manual of botany. Botany. 290 MANUAL OF BOTANY cells or fibres which form part of the soft portions of the vascular bundles of the higher plants [fig. 626). It generally happens that when cell-walls are being thick- ened, the new deposits of cellulose are not laid down uniformly over the surface, but instead form ridges or prominences of various patterns, with unthickened portions between them. The thickening may take the form of a spiral band, winding obliquely round the interior of the cell; it may be deposited in the form of rings placed regularly along its greater axis ; or it may be irregularly laid down, giving rise to particular patterns upon the wall. These forms are known as spiral, annular, and re- ticulated thickenings respectively [figs. 632-634). In other cases the thickening may extend over the greater surface of the wall, leaving only small imthickened spots. Such a cell-wall is said Fig. Fig. 637. Cells whose walls are in px'ocess of tliiekening. The thick and thin plaees on the separating wall correspond in position, p. Pits or thin places, a. Thickening deposit. to "When two cells side by side are undergoing thickening simultaneously, the thick and thin places on the separating wall correspond in position, and the pits appear as delicate canals which in some cases serve as means of communi- cation from cell to cell {figs. 625 and 637). In the cell-walls of the wood-cells of certain trees we lind, in addition to the ordinary pits, large circular discs which encircle them so that each pit looks as if it had a ring surround- ing it (fig. 638) ; hence such cells have been termed cells with bordered pits or disc-bearing wood-cells. This appearance is produced by circular patches of the cell-wall remaining thin after the general thickening has commenced and the rim growing obliquely inwards, leaving ultimately only a narrow orifice in the centre; or, in other words, the opening of the pit into the. Please note that these images
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