ix FILICINEJE LEPTOSPORANGIATM 333 



broad base, more convex on the outer side, and very soon show- 

 ing the circinate vernation. The petiole grows much more rap- 

 idly than the lamina, which remains small until the close of the 

 season before which it unfolds. In most species of colder cli- 

 mates the development of the leaves is very slow, and may oc- 

 cupy 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. strut hiopteris, have pinnate leaves ( Fig. 18 1 , D ) . Their 

 formation is strictly monopodial, and begins by an increase in 

 growth in the outer cells of the young segment, which thus 

 forms a lobe. The marginal cells divide rapidly by longitudinal 

 walls, so that at first the young pinna does not grow from a 

 single apical cell, but sometimes two of the division walls inter- 

 sect and an apical cell is formed. Whether this always happens 

 could not be absolutely determined. As each pinna corresponds 

 to a segment of the apical cell of the leaf, it follows that they 

 alternate with each other on opposite sides of the rachis. 

 Where they grow from an apical cell, the divisions follow 

 those in the apex of the leaf. From the inner cells of the 

 segments the rachis of the pinna is developed. The midrib of 

 each lobe of the pinna bears the same relation to it that the 

 rachis does to the pinna itself. The secondary veins arise in 

 acropetal succession, and at first form a strand of procambium 

 reaching from the midrib to the margin. Where dichotomy of 

 the veins occurs, as it so frequently does in their ends, this is 

 connected with a dichotomy of the marginal group of meriste- 

 matic cells (Sadebeck (6), p. 270). Each marginal cell, like 

 the segment of the apical cell of the leaf, divides into an inner 

 and an outer cell. The latter then divides longitudinally, and 

 the dichotomy is thus inaugurated. These secondary marginal 



