41 



and breaks into more or less rectangular pieces. When in its final 

 stages, it is exceedingly brittle and can be crushed to a fine powder in 

 a mortar. It is always nuich firmer than wood destroj^ed by Polyporus 

 schiminitzii and differs from the latter in the character of the cracks 

 or ])reaks, which are most readily seen on a tangential view. 



The progressive changes which take place in the wood of a Spruce 

 may l)e noted as follows: The wood at first turns slightly red-l>rown 

 in irregular patches, as seen when a trunk is split longitudinally. 

 These patches grow larger, spreading from ring to ring and in a longi- 

 tudinal direction along each ring. Small cracks next appear in these 

 areas, extending part way through the thickness of each ring, both 

 from the side of the spring and of the summer wood. These cracks 

 are ver}- much more visible on the tangential view of an annual ring 

 (PI. XI, fig. 1). At first but scattered cracks are to be seen extending 

 longitudinally^, which, however, soon elongate and pass both diagonally 

 and directly across the direction of the fibers (PI. XI, fig. -I). At this 

 stage the wood is still hard and has acquired a light-brown color. 

 Immediately about the fissures it is more deeply colored than else- 

 where. A microscopic examination shows that there has been great 

 shrinkage in the volume of the cell walls and that the breaks and 

 fissures occur practically throughout the whole mass of the brown 

 wood; though only the larger breaks are visible to the unaided eye. 

 The shrinkage goes on rapidly, and after a time the tension becomes 

 so great that the annual rings separate one from the other. A break 

 usually occurs in a radial direction also, and as a result the free ends 

 of the ring swing outward. Breaks along the lines of the larger 

 medullary rays take place at the same time. This gives rise to long 

 flat slabs of wood, each the width of an annual ring, which hang 

 together loosely at one end and at isolated points on their tangential 

 walls (PI. XIII). Ver}^ badh^ decayed wood is so thoroughl}^ traversed 

 by larger and smaller breaks that it readily falls to pieces when struck. 

 It nuist be noted, however, that the nature of the cracks is such that 

 individual pieces of wood are, as it were, mortised into each other 

 end to end, and this no doubt makes the wood as firm as it is. 



MINUTE CHANGES IN THE WOOD. 



Th(^ minute changes which the mycelium of Polyjyorits Hulfureus indu- 

 ces in the wood cells are such that they can not well be mistaken. It 

 has ))een mentioned that the annual rings break into bands which curve 

 inward as the process of drying goes on. A tangential view of several 

 of these ])ands before they have broken will present an appearance such 

 as is shown on PI. XI, fig. 4. A large number of fissures have formed 

 both across the wood fibers and parallel with them. The latter are more 

 prominent — the cross fissures never occurring alone, ])ut generally con- 

 necting several longitudinal fissures. It will be noted that the breaks are 



