THE PTERIDOPII VTA 



329 



prostrate or subterranean, so that the leaves ai)i)ear to rise directly 

 from the ground. In many tropical species, however, an erect 

 trunk is produced which ])ears a crown of large leaves at its 



Fig. 200. — Transverse section (diagrammatic) of the stem of a fern {Adian- 

 tum). The fibrovascular system is here arranged in a hollow tube, the wood 

 surrounded by bast both within and without. At the right, a segment of the 

 cylinder is passing off to a leaf as a leaf-trace, causing a temporary break, or leaf- 

 gap, in the cylinder. Wood black, bast dotted. 



summit. The fern stem lacks a cambium, and the secondary 

 wood and bast so common in the seed plants are consequently 

 absent. The fibro-vascular system may occasionally be a solid 



Fig. 201. — Fern sporangia of various types. The ring of heavy-walled cells is 

 the annulus, by the contraction of which the wall of the sporangium is ruptured 

 at maturity and its spores scattered. {From Strasburgcr) . 



axis, but is connnonly a i-ing or tube surrountling a central pith 

 (Fig. 200) and is often broken up by gaps into a series of sei)arate 

 bundles. The bast here occurs not only on 1h(> outside of 1h(> 



