IX 



A/A RA TTIA CEAZ—ISOE TA CE^ 



267 



-'I 



-F. 



r - 



impossible to determine, and evidently is not always exactly 

 the same. The root -cap is derived mainly from the outer 

 segments of the apical cell, but also to some extent from the 

 outer cells of the lateral segments ; and the plerome cylinder, 

 where the base of the apical cell is truncate, is formed mainly 

 from the basal segments, 

 but in part as well from 

 the inner cells of the 

 lateral segments. 



The vascular cylinder 

 of the root is usually 

 tetrarch. At four points 

 near the periphery small 

 spiral or annular tracheids 

 appear, and from them the 

 formation of the larger 

 secondary tracheids pro- 

 ceeds toward the centre. 

 The phloem is made up 

 of nearly uniform cells 

 with moderately thick 

 colourlesswalls, A bundle- 

 sheath is not clearly to be 

 made out (Fig. 137). 



The cotyledon is des- 

 titute of the stipules found 

 in the perfect leaves of the 

 Marattiacea:;, but they are 

 well developed in the third 

 leaf, where they form two 

 conspicuous appendages 

 clasping the base of the 



next VOUnc>"eSt leaf The ^"^- ^39-— ^^«''«'^'«-0<'2'^/«-s-" (Baker), a, Longitudinal 



■^ "^ _ ' section of the young sporophyte, showing the distri- 



edgeS of these stipules are bution of the vascular bundles, X6; /, leaves; s(, 



cjompwhal- ^prr3tp anH tViP stem apex ; r, a root ; y; the foot ; B, young sporo- 



somewnat serrate, ana tne p^yte with the prothaiuum (/r), stiii persisting. 

 edges of the two meet. 



much like two bivalve shells. The strictly dichotomous 

 character of the cotyledon is gradually replaced in the later 

 leaves by the pinnate arrangement, both of the divisions of the 

 leaf and the venation. This is brought about in both cases by 

 an unequal dichotomy, by which one branch develops more 



