. The structure and development of mosses and ferns (Archegoniatae). Plant morphology; Mosses; Ferns. 338 MOSSES AND FERNS CHAP. primary tracheids are formed a group of similar ones, and from here the formation proceeds towards the central group of large tracheids, which are the last to have their walls thickened and lignified. The large secondary tracheids are scalariform, like those of the stem. The cells of the pericycle remain nearly unchanged, but in the two phloem masses, according to Poir- ault (I) sieve-tubes are always present. These tubes are of two types, those with horizontal trans
. The structure and development of mosses and ferns (Archegoniatae). Plant morphology; Mosses; Ferns. 338 MOSSES AND FERNS CHAP. primary tracheids are formed a group of similar ones, and from here the formation proceeds towards the central group of large tracheids, which are the last to have their walls thickened and lignified. The large secondary tracheids are scalariform, like those of the stem. The cells of the pericycle remain nearly unchanged, but in the two phloem masses, according to Poir- ault (I) sieve-tubes are always present. These tubes are of two types, those with horizontal transverse walls, and those with inclined ones. The perforations in the sieve-plates were. Fig. i88.—Adtantum emarginafum. A, Longitudinal; B-E, a series of transverse sec- tions of the root, X^oo; x, apical cell; s-s, sextant walls; en, endodermis. demonstrated, and lateral perforations, either isolated or in groups, also occur. His statement that the sieve-tubes have no nuclei requires further proof. The walls of the sieve-tubes are of cellulose, but in the sieve-plates callus is found. The rest of the phloem is composed of conducting cells, with thin walls and oblique septa. The endodermis often becomes dark-coloured and its walls lignified, and when the root dries the vascular cylinder becomes separated from the ground tissue by the trans- verse splitting of the endodermal Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Campbell, Douglas Houghton, 1859-1953. New York, The Macmillan Company;
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