362 



GENERAL MORPHOLOGY OF PLANTS 



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

 edges of all of the perpendicular cell walls, so that 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 straighten- 

 ing 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. 322). 



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

 provided with a well-developed fibre-vascular system which 



Fig. 323. 



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. 



Fig. 324- 



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 100, 104). 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 

 arrangement (fig. 323). While the bundles strengthen the stems 

 somewhat, they are comparatively weak, and in many fern stems 



