82 VEGETABLE HISTOLOGY. 



corrosive fluid the middle bark. Then comes a narrow strand 

 of deeply-stained cells, which on the ends of the section may be 

 seen to be fibrous, and also farther in if the section be not too 

 thick, in which case there may be observed how the fibres are 

 spliced over one another. Next to the bast is another colorless, 

 narrow layer composed of the soft bast and cambium. Adja- 

 cent to this is the broader strand of green wood fibres and 

 vessels. 



Now tap gently with a teasing needle on the cover glass 

 directly over the sections. The macerating fluid has dissolved 

 the middle lamellas, so that by tapping the cells spread apart 

 and are easily studied individually. The fibres are very long, 

 the wood fibres being often as much as 20 or 30 times as long 

 as wide and the bast fibres even longer; the ends taper to a 

 point, sometimes, however, abruptly, and at times are forked. 



Fig. 49. Longitudinal view of fibres from stem of Geranium (Bastln). 



The walls are smooth, and with careful staining and good light 

 delicate slits are seen running obliquely across the walls. Fig. 

 49 illustrates both bast and wood fibres in longitudinal section. 

 Sometimes bast and wood fibres are found which are made up 

 of a row of two or three cells and then they have cross-partitions 

 in the lumen or cavity of the fibre. 



BAST FIBRES OF CINCHONA BARK. 



Bast fibres vary greatly in length and lignification. There 

 are some extremely long, flexible, slightly lignified fibres, for 

 example, Flax, Hemp, Mezereum, on one extreme, and very 

 short, brittle, much lignified fibres on the other extreme, as 

 found in Cinchona bark. 



Make cross and longitudinal radial sections of Cinchona bark 

 (calisaya or succirubra) that has been softened in water or 

 alkali. 



Examine first a cross-section with low power. In the inner 

 bark there are numerous yellowish, solid-looking quadrangular 

 cells, the cavities of which have been reduced nearly to a point 

 by the great thickening of the cell-walls. These are the bast 

 fibres. They occur singly or in small radial groups of three or 

 more. Incidentally, the student should note carefully the 

 structure of the whole section, as an example of a typical well- 

 developed bark. On the outer edge is a thick layer of small 

 cork cells, deeply colored with natural plant pigment. These 

 have a tendency to flake off in places and are divided here and 



