42 BULLETIN 80, U. S. DEPARTMENT OF AGRICULTURE. 
Determinations 1 made on sound sticks of aspen varying from 8 to 10 inches in diame- 
ter showed about 62.8 per cent of cellulose. Miiller, quoted by Clapperton, 2 gives the 
following analysis 3 for the poplars: 
Per cent. 
Cellulose 62. 77 
Resin 1. 37 
Aqueous extract 2. 88 
Water 12. 10 
Lignin 20. 88 
100. 00 
Since bleached pulp is very nearly pure cellulose, the maximum yield obtainable 
could not be appreciably higher than 63 per cent. 
Aspen wood is made up of three types of structural elements — fibers, vessels, and 
parenchymatous tissue. The latter comprises the medullary ray cells and the rather 
scantily developed parenchyma cells at the end of the year's growth. The structure 
is shown in Plates YIII and IX. In Plate VIII, figure 1, the long tubes running the 
length of the picture are the vessels; cross sections of the medullary rays can be seen 
scattered among the fibers as dark vertical "plates," one cell in width and several in 
height. The ray cells are characterized by exceedingly thin walls, and when the 
wood is cooked for pulp these cells readily dissolve. The vessels are more resistant to 
chemical attack than the parenchymatous tissue, and the fibers, because of their 
relatively thick walls, are least affected by the cooking process. It is also possible 
that the cellulose constituting the fiber walls is more resistant than the cellulose of 
the other elements. In Plate IX, figure 2, the middle lamella or intercellular sub- 
stance appears as a black line between the adjacent walls of the elements. This is 
dissolved in the process of cooking for paper pulp. 
Aspen fibers are comparatively short. Examples of long-fibered woods used in" 
paper making are spruce, hemlock, and balsam fir, and of medium-length ones tulip 
tree, sweet gum, and cottonwood. The actual dimensions of aspen fibers vary a 
great deal with the tree and the part of the tree from which secured. Forest Service 
measurements 4 of a large number of fibers of aspen wood showed a range of from about 
0.5 to 1.6 mm. in length and an average length of 1.0 mm. 5 
PULP WOOD CONSUMED. 
At the present time soda, sulphite, sulphate, and mechanical pulps are made from 
aspen and other poplars, but the soda process has always used these woods in by far 
the greater amounts, and they continue to form the chief pulpwood supply for this 
process. The other processes of pulp making have been applied to the poplars within 
recent years only, although it was known 20 or 30 years ago that they could be ground 
for mechanical pulp and could be reduced without difficulty by the sulphite process 
to an easy-bleaching pulp. The properties of the wood and the yields and qualities 
of the pulp made from it, combined with the proximity of an adequate supply and 
its relatively low cost, made this the best wood obtainable for the manufacture of soft, 
easy-bleaching soda pulp. 
i Forest Service Bulletin 93, p. 7, 1911. 
* Practical Papermaking, p. 43, 1907. 
3 This analysis makes no mention of the ash. According to Sargent (Tenth U. S. Census Rept., Vol. IX) 
the ash in aspen varies from 0.31 to 0.76 per cent, with an average of 0.55 per cent, of the air-dry wood. See 
this report also for further data on the chemical composition and properties of aspen. 
* Forest Service Bulletin 93, p. 7, 1911. 
5 One millimeter is equivalent to approximately one twenty-fifth of an incn. 
