36 BULLETIN 1136, U. §. DEPARTMENT OF AGRICULTURE 
sapwood and heartwood, and by various other irregularities in struc- 
ture and in drying. (Pl. 13.) Loosening of knots is caused by the 
drying-out or the exudation of cementing resins and gums with dead 
knots, and with live knots by the differences in shrinkage resulting 
from the right-angle relation of the axes of the knot and of the tree— 
the axis of the knot coincides with that of its branch. The knot 
shrinks away from the rest of the wood lengthwise of the board, 
but does not do so appreciably in the crosswise direction. End 
checking, which is caused by the excessively rapid drying of the end 
surfaces, is discussed more fully under “ Drying schedules.” 
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Fictrp 7.—Development of a surface check into a honeycomb. In stages 1, 2, and 3 
the check is gradually closing as the center of the piece shrinks in drying. Stages 4, 
5, and 6 show how the tensile stresses deepen the honeycomb as the casehardening 
pEcomes more severe. The depression above the honeycomb in stages 5 and 6 is 
ypica 
COLLAPSE 
One form of seasoning defect is the actual collapse of rows of cells, 
just as a rubber tire collapses when the air is let out. (PI. 14, A.) 
This defect occurs principally in the heartwood of only a few species, 
such as redwood, western red cedar, cypress, swamp-grown oaks, and 
red gum, although it is occasionally seen in other species, such as 
hickory and black walnut. No entirely successful method of avoiding 
it has been developed. The establishment of a steep moisture gra- 
dient at the beginning of the drying, however, is of much assistance. 
Such a gradient may be secured by the use of low humidities, but 
they may have to be accompanied by low temperatures to prevent 
injury to the stock. In some cases sumply the use of low tempera- 
tures at the beginning of the run, without unusually low humidities, 
seems sufficient. 
