OP ARTS AND SCIENCES. 



205 



ally as many as four to six cells in thickness. The very central mass, 

 both of the medulla of the stipe and of the medulla of the blade, con- 

 sists of densely interwoven hyplia3, which have made their way in 

 from the surrounding cells, and have crowded the tubes out to the 

 periphery. 



In the very old stipes, and to a somewhat less degree in the blades, 

 the tubes form an important factor in rendering them as tough and 

 leathery as they are. When such fi'onds are macerated in potash, 

 the limiting and cortical layers may be removed, and a firm mass 

 of fibrous tissue left, which is in the form of a fiat plate in the blade, 

 while that from the stipe is a cylinder hollow or spongy in the 

 middle. This layer is unbroken at the transition place, which shows 

 that in the adult plants this region is no longer the seat of active 

 growth. 



It is easy to isolate individual fibres from macerated specimens to 

 get a better idea of their shape and size. They have, without ex- 

 ception, a longitudinal course, very nearly vertical, and lie closely 

 crowded with overlapping ends. They are for the most part simple, 

 but some are slightly branched at one end. 



In a macerated stipe 9.3 cm. long, the longest fibre (or thickened 

 tubular cell) was 7.5 cm. long, and was terminated only at one end, 

 the other being broken off short. A whole one obtained from the 

 same stipe was 7.3 cm. long, and one of more average length was 

 6.2 cm. long. A ver}' small one measured 3.2 cm. The fibres from 

 the blade are much shorter, averaging from 3 to 4 cm. in length. 



In Figure 25, a, b, c, are represented the tips of various fibres as 

 seen in optical section. They are of various shapes, and frequently 

 have spiny projections which are probably traces of the connecting 

 hyphae with which in their earlier stages they were united with 

 adjacent cells. In June plants, they have a thickened and stratified 

 wall and a wide lumen. The contents at this time also line the 

 wall, and are dense and of a yellow color. But in the specimens 

 figured, which are from an October plant, the lumen is almost 

 obliterated. The walls are composed of very distinct layers. The 

 treatment with potash and a prolonged soaking in water are respon- 

 sible for some of this, but not for the greater portion. The walls 

 when treated with chloriodide of zinc solution finally become a pale 

 watery blue. 



These fibres, both as regards shape and arrangement, remind one 

 of the fibres in the bands of brown tissue so common in tlie stems and 

 stipes of ferns which are called sclerencfiyma fibres. There seems to 



