. The structure and development of mosses and ferns (Archegoniatae). Fig. 187.—Pteris cretica. Origin of lateralrootlet from the endodermis of the root; en,endodermis of the main root; x, apical cellof the rootlet; p, digestive pouch. (AfterVan Tieghem.) 22 338 MOSSES AND FERNS CHAP. primary tracheids are formed a group of similar ones, and fromhere the formation proceeds towards the central group of largetracheids, which are the last to have their walls thickened andlignified. The large secondary tracheids are scalariform, likethose of the stem. The cells of the pericycle remain nearlyunchang
. The structure and development of mosses and ferns (Archegoniatae). Fig. 187.—Pteris cretica. Origin of lateralrootlet from the endodermis of the root; en,endodermis of the main root; x, apical cellof the rootlet; p, digestive pouch. (AfterVan Tieghem.) 22 338 MOSSES AND FERNS CHAP. primary tracheids are formed a group of similar ones, and fromhere the formation proceeds towards the central group of largetracheids, which are the last to have their walls thickened andlignified. The large secondary tracheids are scalariform, likethose of the stem. The cells of the pericycle remain nearlyunchanged, but in the two phloem masses, according to Poir-ault (i) sieve-tubes are always present. These tubes are oftwo types, those with horizontal transverse walls, and thosewith inclined ones. The perforations in the sieve-plates were. Fig. i88.—Adiantum emarginatum. A, Longitudinal; B-E, a series of transverse sec-tions of the root, X200; x, apical cell; s-s, sextant walls; en, endodermis. demonstrated, and lateral perforations, either isolated or ingroups, also occur. His statement that the sieve-tubes have nonuclei requires further proof. The walls of the sieve-tubes areof cellulose, but in the sieve-plates callus is found. The rest ofthe phloem is composed of conducting cells, with thin walls andoblique septa. The endodermis often becomes dark-colouredand its walls lignified, and when the root dries the vascularcyhnder becomes separated from the ground tissue by the trans-verse splitting of the endodermal cells. IX FILICINEAi LEPTOSPORANGIAT^ 339 The secondary roots arise in regular succession in two lines,corresponding to the ends of the xylem plate in the diarchbundle. They themselves generally branch further, and thusvery extensive root systems are formed. The origin of thelateral roots of the Ferns has bee
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