42 BULLETIN 80, U. S. DEPARTMENT OF AGRICULTURE. 



Determinations ' 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 VIII 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 



PULPWOOD 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. 



2 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. 



'- One millimeter is equivalent to approximately one twenty-fifth of an inch. 



