1920] 
ZELLER—IMBIBITION BY WOOD AND SPORE GERMINATION 65 
absorption of moisture by certain morphological and mechanical 
properties of the woody tissues which vary with the different 
species of wood. This variation in morphological and mechani- 
cal characters in the wood of different species will account for 
the slight differences in moisture content at the fibre-saturation 
point of shortleaf and longleaf specimens. The moisture con- 
tent at the so-called fibre saturation point of the four*curves is 
approximately as follows: shortleaf pine sap-wood, 24.25 per 
cent, heart-wood, 24.5 per cent, and longleaf pine sap-wood, 
23.75 per cent, and the heart-wood, 23.25 per cent. 
In timber-testing laboratories the fibre-saturation point of 
wood has been determined by means of strength tests, and has 
been defined by Tiemann (707) as “the degree of moisture at 
which maximum absorption by the cell walls is reached." After 
this point is reached added moisture does not lessen the strength. 
Beginning with the dry conditions, with the increase of moisture 
the strength falls off very rapidly at first, then more slowly as 
the fibre-saturation point is reached, and here it abruptly ceases 
to decrease. This abrupt break in the moisture-strength rela- 
tion represents the fibre-saturation point. The moisture per 
cent at fibre-saturation for longleaf pine as determined by Tie- 
mann ('07) by compression tests on small specimens averaged 
25 per cent, with a maximum at 26 per cent and a minimum at 
24 per cent. This is not far removed from the moisture content 
at which the curves representing the three different specific 
gravities of longleaf pine diverge in figs. 1 and 2. 
If this divergence of the moisture-humidity curve represents 
` the fibre-saturation point the appreciable increase in moisture 
up to 100 per cent humidity must be moisture in the form of a 
surface film; that is, the fibre-saturation point is the point where 
absorption or imbibition by the fibre is replaced by a surface 
phenomenon, adsorption. In a unit weight of wood fibre the 
thin-walled, large-lumened cells, having a lower specific gravity 
than the heavy-walled, small-lumened cells, present a much 
greater surface than the latter. If the greater concave curva- 
ture of the smaller-Iumened cells has any tendency to thicken the 
surface film in proportion to that adsorbed by those having less 
curvature, the difference in the total moisture created in this 
way evidently is not great enough to overcome the difference 
resulting from the difference in surface. 
5 
