. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. THE PRIMARY PERMANENT TISSUES 37. The contents of the sieve tubes are found to be rich in pro- teids, amido-acids, and soluble carbohydrates, and minute starch grains may sometimes be present in abundance. Even proteids that are in solution do not pass readily through cell-walls, and in the sieve tubes the perforations allow them to pass in an unob- structed stream from one cell or sieve tube member to the other. That the sieve tubes are for the vertical flow of pr
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. THE PRIMARY PERMANENT TISSUES 37. The contents of the sieve tubes are found to be rich in pro- teids, amido-acids, and soluble carbohydrates, and minute starch grains may sometimes be present in abundance. Even proteids that are in solution do not pass readily through cell-walls, and in the sieve tubes the perforations allow them to pass in an unob- structed stream from one cell or sieve tube member to the other. That the sieve tubes are for the vertical flow of proteids and allied substances is shown by direct ob- servation under the microscope while using suitable reagents for the demonstration of proteids; and further by girdling and constric- tion experiments described in Chapter X. The sieve parenchyma cells in differentiating from the procam- bium elongate vertically more or less and increase in their cross diameters (Fig. i8), but they do not become, as a rule, so large in any dimension as the cells of the sieve tubes. Their walls remain cellulose and commonly thicken but little. They appear to serve chiefly in the translocation of car- bohydrates and as storage places for proteids which they are in position to take frolm the sieve tubes when a surplus is at hand, and they assist in delivering over to the medullary rays materials from the sieve tubes to be stored by the rays or transported inward for storage in the cells of the wood or xylem parenchyma. The sieve tubes, companion cells, and sieve parenchyma cells seem to remain alive and functional throughout the first year of Fig. i8.—^Stages in the development of sieve tubes, companion cells, and phloem parenchyma. A, a and b, two rows of procambial cells; in c and d, a has divided longitudinally and c is to become companion cells; d, a sieve tube, and b, phloem paren- chyma; B, c, companion cells, and d, a beginning sieve tube from c and d, respectively in A. The cross-walls
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