160 ELEMENTS OF APPLIED MICROSCOPY. 
s. The Commoner Paper Fibres.—With a few of the 
most important materials of paper the student should 
thoroughly familiarize 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 Coniferee—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 
Fic. 39.—TRACHEID OF ACONIFER. (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 
