100 ELEMENTS OF APPLIED MICROSCOPY. 



5. The Commoner Paper Fibres. — With a few of the 

 most important materials of paper the student should 

 thoroughly famiUarize himself. The fibres of cotton, 

 linen, hemp, manila, jute, and silk have already been 

 treated with some fulness in Chapter VII. Of the woods 

 used in the manufacture of paper pulp there are two dis- 

 tinct types. The Coniferas — spruce, fir, pine, etc. — ex- 

 hibit cells of markedly different structure from the Angio- 

 sperms, of which poplar and birch are the most impor- 

 tant examples. Finally, straw and esparto grass are 

 distinguished by a third characteristic type of tissue. 



6. The Structure of the Gymnosperms. — ^Wood-pulp 

 made from the spruce, fir, balsam, larch, or hemlock is 



Fig. 39. — ^Tracheid of a Conifee. (After Herzberg.) 240 diameters. 



mainly made up of the cells from the tracheids or fibro- 

 vascular bundles of the stem. In all the trees of this 

 group the structure is essentially the same. The cells 

 are long, exceeding the diameter of the low-power field, 

 and of considerable breadth (see Fig. 39). - The ends 

 are often contracted to a rather acute point. Sharp 

 twists are sometimes present, due to harsh treatment in 

 the preparation of the fibre. The characteristic feature 

 of these cells, however, is the presence of numerous round 



