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 



D 



FIG. 188. Adiantum emarginatum. 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 cells. 



