FIBRES. 



83 



there by transverse clefts. Next to the cork is the middle bark, 

 consisting of lighter colored, tangentially elongated paren- 

 chyma cells. In it is a tangential row of secretion channels. 

 Next to the middle bark is the inner or bast bark, which is 

 divided into compartments by distinct radial lines of cells, two 

 or three rows wide, which are the medullary rays. Between 

 these rays occur the bast fibres and a tangled network of 

 smaller thin-walled cells, consisting of ordinary parenchyma 

 and sieve tubes, the latter not being easily made out. 



Under high power the excessively thick- 

 ened walls of the bast fibres are made up of 

 layers which are finely stratified. The walls 

 are also crossed by radiating fine canals 

 (Fig. 50) . In a section stained with methyl- 

 green solution the walls are seen to be 

 strongly lignified, but more so in the ex- 

 terior layers than in those next to the cavity, 

 as is shown by the different depths of color. 



LONGITUDINAL SECTION. In this the cork 

 and parenchyma look pretty much as in 

 cross-section, but the secretion space, if the 

 section contain one, appears as a long, empty 

 channel, and the medullary rays appear as 

 isolated plates of cells lying across the sec- 

 tion. The bast fibres, which are seen best 

 at the ends of the section, are relatively 

 very short, being about six times as long as 

 wide, fusiform or wedge-shaped on the ends. 

 The cavity appears as a line and the strati- 

 fication is distinct. The fibers are also 

 marked by cross-lines radiating from the 

 cavity (Fig. 50). 



Macerate a section in Schulze's solution, 

 as in the case of the Geranium section, wash, 

 mount and separate by tapping on the cover glass. The bast 

 fibres are separated and may be studied more easily. Stain a 

 macerated section in methyl-green, tap out on a slide and note 

 the effect of the stain. 



Fig. 50. Bast fibres of 

 Cinchona calisaya in 

 cross and longitudi- 

 nal section, showing 

 narrow cavity, pore 

 canals and stratifi- 

 cation of walls (re- 

 duced, from Bastin) . 



