X FILICINEJC LEPTOSPORANGTAT^E 325 



less regular than in the trachcids, and the bars more or less 

 anastomosing so as to enclose thin areas, the sieve-plates (Fig. 

 163, C). These occur all over the lateral walls, as well as the 

 transverse ones. While it could not be positively shown, it is 

 extremely probable that the pores, afterwards formed, penetrate 

 completely the thin membrane of the sieve-plates, and throw 

 the adjacent sieve-tubes into communication. 



The Leaf 



While the leaf in a few of the Leptosporangiatae is simple, 

 in much the larger number it is compound, either dichotomously 

 branched {Adzantiiin pedatinii) or more commonly pinnately 

 divided. Owing to the great irregularity of the divisions and 

 slow formation of new segments in the stem apex, it is 

 exceedingly difficult to determine positively whether each 

 segment of the stem apex produces a leaf, but this seems 

 probable. The leaf appears as a blunt conical emergence, 

 whose apex is occupied by a single large apical cell, which in 

 nearly all forms examined is wedge-shaped and forms two rows 

 of segments. As the leaf grows it assumes the form of a 

 flattened cone with a broad base, more convex on the outer 

 side, and very soon showing the circinate vernation. The 

 petiole grows much more rapidly than the lamina, which remains 

 small until the close of the season before which it unfolds. In 

 most species of colder climates the development of the leaves 

 is very slow, and may occupy three or four years. The last 

 stage of growth consists merely in an expansion of the leaf, 

 with comparatively little cell division. This latter phase of 

 growth often goes on with great rapidity, in strong contrast to 

 the excessively slow growth during the early stages. 



The first wall in the young segment of the apical cell 

 divides it into an inner and an outer cell, and the latter then 

 divides into two by a longitudinal wall, and each of the latter 

 into two more by a transverse wall. Of these five cells, the 

 inner ones, in the lamina of the leaf, produce the rachis, the 

 outer ones the lamina itself. The outer cells of the segments 

 form the pinnae. Soon after the separation into lamina and 

 petiole, the development of pinnae begins in those Ferns which, 

 like O. struthiopieris, have pinnate leaves (Fig. 162, D). Their 

 formation is strictly monopodial, and begins by an increase in 



