332 



GENERAL MORPHOLOGY OF PLANTS 



branous are pressed inward. This exerts a pull on the outer 

 edges of all of the perpendicular cell walls, which act as so many 

 fulcra, and the outer edges of these different walls are brought 

 nearer and nearer together. This causes the inner walls to 

 curve slowly outward in unison. The result is that the entire 

 ring begins to straighten out. The lower end is held firmly at 

 the base of the spore case. The lip cells are torn apart, the 

 upper one being raised by the straightening ring. At the same 

 time, the two lateral faces of the spore case are torn across, and 

 as the ring curves backward it carries with it the upper half of 

 the spore case and nearly all the spores. When the ring has 

 curved back so that it is almost doubled on itself, it suddenly 

 snaps back again nearly to its former position, and scatters the 

 spores (fig. 308). 



502. Structure of the fern stem. The stems of ferns are 

 provided with a well-developed fibro-vascular system which 



Fig. 309. 



Concentric bundle from stem of Polypodium 

 vulgare. Xylem in the center, surrounded by 

 phloem, and this by the endodermis. (From 

 the author's Biology of Ferns.) 



par. 



62 



Fig. 310. 



Section of stem (rhizome) of Pteris 

 aquilina. sc, thick-walled scleren- 

 chyma; a, thin-walled sclerenchyma; 

 par, parenchyma. 



serves for transport of water and also strengthens the stems. 

 The bundles are, however, usually concentric instead of collateral as 

 in the higher plants (see paragraphs 94, 98). The wood is in the 

 center and is surrounded by the bast, the bast by the bast sheath, 

 and this by the endodermis, thus giving a concentric arrange- 

 ment (fig. 309). While the bundles strengthen the stems, they 

 are comparatively weak, and in many fern stems there are large 



