462 MOSSES AND FERNS 



CHAP. 



large species, like E. tchnateia, it is difficult to trace any such 

 relation. In most forms, by subsequent dichotomy of some or 

 all of the primary teeth, others are formed, so that the number 

 in the fully-developed sheath exceeds that first formed. As 

 soon as the young sheath begins to project, a section through 

 one of the teeth shows that it is divided into an upper and lower 

 tier of cells, the apical cell terminating the upper one. This 

 division no doubt corresponds to the first horizontal division in 

 the outer nodal cell from which the leaf-tooth originally comes. 

 In one a little older (Fig. 272, B), in this upper tier of cells a 

 line of cells occupying the axis is evident (fb), extending from 

 the base of the leaf nearly to the summit, and growing at its 

 outer end by the addition of cells derived from the inner part of 

 the youngest upper segments of the terminal cell of the leaf. 1 

 This is the beginning of the single vascular bundle found in each 

 leaf. 



Shortly after this first indication of the vascular bundle of 

 the leaf can be seen, the cells of the cortex immediately outside 

 the central pith begin to divide rapidly by longitudinal walls and 

 form a zone of cambiform cells completely surrounding the 

 medulla. In the primary central row of cells in the leaves 

 similar divisions occur, and a very evident procambium cylinder 

 is formed, bending in and joining the procambium zone of the 

 cortex. At the point of junction the cells are shorter and 

 broader, and the cortical cells lying outside are also much 

 broader, so that the cortical procambium is very conspicuous. 

 If cross-sections are examined about this time, in the procam- 

 bium zone are found a number of groups of cells where the 

 divisions are more rapid, and the resulting cells narrower than 

 the surrounding ones. These are the separate vascular bundles, 

 and are continuous with those in the leaves (Fig. 269). The 

 first permanent tissue consists of one or two small annular 

 tracheids upon the inner side of the bundle (Fig. 268, C). 

 These are followed by several others. They first form in the 

 internodal part of the bundle and only later in the foliar portion. 

 The nodal tracheids joining the xylem of the foliar and inter- 

 nodal bundles are very irregular short cells with annular thick- 

 enings upon their walls. Later two small groups of larger 

 spiral tracheae are formed at the sides of the xylem, but the 



1 Each tooth is here regarded as a leaf, the sheath as a circle of con- 

 fluent leaves. 



