244 MOSSES AND FERNS chap. 



marked pith. The vascular ring is broken by a gap above the 

 first leaf-trace (cotyledonary stele), and the pith is thus thrown 

 into communication with the outer ground tissue, or cortex. 



The first tracheary tissue appears shortly after the root has 

 broken through the calyptra, at which time the root has the 

 length of 5-20 millimetres. The development of the tracheary 

 tissue in the root begins at two, or more commonly three, 

 points, i. c, the root is either ''diarch" or ''triarch." The in- 

 nermost layer of the fundamental tissue forms the ''endoder- 

 mis" or bundle-sheath. As is usually the case, the endodermal 

 cells are characterised by the peculiar thickening or foldings of 

 the radial walls, which appear as elongated dots in transverse 

 sections. A similar endodermis can be made out, surrounding 

 the stelar tube of the stem. 



The primary tracheids, or ''protoxylem," have reticulately 

 sculptured walls, and, except in size, closely resemble the secon- 

 dary tracheary elements, or ''metaxylem," which are formed 

 centripetally, and meet in the centre of the vascular cylinder. 

 Between the xylem masses are as many masses of phloem, or 

 bast, made up in part of sieve-tubes with which are mingled 

 elongated paranchyma cells. Surrounding the circle of xylem 

 and phloem masses is the pericycle, composed of one or two 

 layers of parenchyma. 



After the young root has broken through the calyptra and 

 penetrated the ground, the cotyledon grows upward and finally 

 makes its appearance above the surface of the ground. It 

 becomes differentiated into a slender, nearly cylindrical stalk 

 (stipe) and a much-divided lamina (Fig. 127, E). The single 

 primary vascular bundle of the leaf-rudiment divides into two 

 within the stalk, and passes into the two lateral lobes of the 

 lamina. From one of tliem a strong branch is developed which 

 constitutes the midrib of the central segment of the lamina. 

 The vascular bundles of the stipe approach the collateral type, 

 rather than the concentric structure found in the later formed 

 leaves. 



Sometimes two or three roots are developed before the 

 cotyledon unfolds, and the young sporophyte remains for a long 

 time — probably two or three years — attached to the gameto- 

 phyte, the superficial cells of the foot remaining active during 

 this period. These cells show the dense cytoplasm and con- 

 soicuous nuclei of active cells. 



