PTEltlDOPHYTA 



257 



Ferns. Two strands of much smaller tracheids, with spiral or retic- 

 ulate thickenings, occupy the foci of the elliptical section. These 

 are the primary tracheids, " Protoxylem," and from these the devel- 

 opment of the tracheary tissue proceeds centripetal ly. 



The phloem, which completely surrounds the xylem, is composed 

 of elongated walled cells, some of which are developed into sieve- 

 tubes. These have numerous sieve-plates upon their lateral walls. 

 Vessels, i.e., tracheary elements composed of several fused cells, are 

 rare in the Ferns. 



The Leaf 



Where the stem is prostrate, leaves are developed upon the dorsal 

 side only. Where it is upright, the leaves usually form a crown at 

 its summit. In their early stages, 

 the growth of the leaf is usually 

 apical, generally from a two-sided 

 cell. In Osmunda the apical cell 

 of the young leaf is tetrahedral. 

 Later the growth is chiefly basal. 

 The segmentation of the apical 

 <jell is much like that of the .stem, 

 and the separation of the primary 

 tissues is accomplished in much 

 the same way, and takes place 

 very early. 



The leaves in most young Ferns 

 are dichotomously branched, but 

 this is not usually the case in the 

 mature leaf, although it may be ; 

 e.g., species of Gleichenia, Tri- 

 chomanes, Adiantum pedatum, etc. 

 Much more commonly the leaves 

 are pinnately divided, and the 

 branching is monopodial, the primary divisions corresponding to the 

 two series of segments of the apical cell. 



The growth of the leaf is very slow in many Ferns, especially 

 those of cooler regions, where it often takes three years for the com- 

 plete development of a single leaf. A section through the apex of 

 the stem of such a Fern will show two, and sometimes three, complete 

 series of leaves, representing as many seasons' growth. The lamina 

 remains rudimentary until the season preceding its expansion, when 

 it rapidly develops, so that it is completely formed by the end of the 

 growing season, and is ready to expand very quickly in the following 

 spring. This accounts for the extraordinary rapidity with which the 

 leaves of many Ferns expand in the spring or early summer. 



FIG. 222. A, B, Struthiopteris Ger- 

 manica; A, tirst; B, second, leaf of 

 young sporophyte, showing dichoto- 

 mous venation ( X 3) . C, Woodwardia 

 radicans, areolated venation (X 2). 



