X FILICINE.E LEPTOSPORANGIAT.K 323 



perpendicular to its broad faces. Each of the six semi-segments 

 is then divided into an inner and an outer cell, and the latter 

 again by a longitudinal wall parallel to its inner and outer faces, 

 so that each original segment is divided into two inner cells 

 and four outer ones. From the inner cells the pith and vascular 

 bundles arise, from the outer ones the cortex and epidermis, 

 but after the first divisions there is great irregularity in the 

 succession of the cells. The young vascular bundles can be 

 traced nearly to the apex, and first appear as bundles of 

 procambium cells, which lower down unite and are joined by 

 others from the leaves and roots. 



In (9. strutJiiopteris characteristic air-chambers are formed 

 in the young medulla at an early period. At certain points 

 the cells become longer and their contents more transparent. 

 These cells divide less rapidly than the surrounding tissue, and 

 large intercellular spaces are formed. The loose cells about 

 these form masses of trichomes, either hairs or scales, which 

 later dry up and leave a large empty space, which may or may 

 not communicate with the exterior through the foliar gaps. 



In Onoclea striithioptcris, as in most leptosporangiate Ferns, 

 the outer cortical cells become changed into sclerenchyma. 

 In 0. strutJiiopteris the sclerenchyma forms several hypodermal 

 layers, distinctly separated from the inner cortical parenchyma. 

 These sclerenchyma cells are much elongated ; their lateral 

 walls are somewhat uneven, and in their younger stages swell 

 up more strongly under the action of potassic hydrate than do 

 the cortical cells. Their walls become thick, are first pale 

 yellow, and later a dark reddish brown. The walls are very 

 markedly striate, and the central lamella distinct. Deep pits 

 extend down to the latter. 



The bundles in the stems of the Polypodiaces are very 

 uniform in structure. They are usually elliptical in section, 

 and the first tracheary tissue formed is a strand of small spiral 

 or reticulate tracheids at the foci of the bundle. From there 

 the formation of the very large scalariform ones, so characteristic 

 of the leptosporangiate Ferns, proceeds towards the centre of 

 the bundle, where the last -formed ones are situated. The 

 young tracheids have thin walls and abundant protoplasm, but 

 as the wall thickens, the contents gradually disappear, and 

 finally no living protoplasm remains in them. Faint elongated 

 transverse pits become evident, and the spaces between these 



