IX FILICINE^ LEPTOSPORANGIAT^ 329 



are formed that are quite different at first in appearance from 

 the ordinary shoots. The main axis in this species is an 

 upright rhizome about 2 cm. in diameter, but appearing much 

 larger on account of the thick persistent leaf-bases which cover 

 it. The stolons arise from the bases of these leaves, apparently 

 as adventitious buds. They may remain dormant for a long 

 time, as very many more of the very small ones are found than 

 those that are fully developed. They finally bend upward, 

 and the scattered scale-like leaves give place to the perfect green 

 ones. The main rhizome is occupied by a central cylinder com- 

 posed of a network of anastomosing bundles. Inside of this 

 cylinder is a medulla made up of large parenchyma cells, and 

 communicating with the cortex by means of the foliar gaps, or 

 spaces between the bundles. 



Fig. 181, A shows a longitudinal section of the apex of a 

 stem of Adiantum emarginatum, which shows the typical ap- 

 pearance in the Polypodiacese. The apex of the stem forms a 

 slight cone, whose centre is occupied by the large initial cell, 

 which is deeper than broad. In cross-section it shows much 

 the same form. Divisions occur, evidently, only at compara- 

 tively long intervals, and each segment presumably gives rise to 

 a leaf. The first division in each segment is longitudinal and 

 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 pro- 

 cambium cells, which lower down unite and are joined by others 

 from the leaves and roots. 



In O. struthiopteris 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, w^hich 

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

 not communicate with the exterior through the foliar gaps. 



