460 FUNGOUS DISEASES OF PLANTS 



brown and, to a considerable extent, separated into plate-like areas, 

 corresponding in their radial diameters to the seasonal wood rings. 

 These plates are subsequently broken into smaller areas by lateral 

 contact, and in all of the clefts thus formed by the processes indi- 

 cated, the mycelium often grows (especially in deciduous trees) in 

 sheath-like strata, the particular appearance of the mycelium, how- 

 ever, being modified in different hosts, largely depending upon the 

 density of the wood (Fig. 227). In all cases the wood is brittle in 

 the last stages of decay and may be readily reduced to a powder. 



According to von Schrenk the detailed changes induced in the 

 wood of the spruce may be stated as follows : 



Minute changes in the wood. 1 The minute changes which the mycelium of 

 Polyporus sulfureus induces in the wood cells are such that they cannot be 

 mistaken. It has been 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 bands 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, but generally connecting several longi- 

 tudinal fissures. It will be noted that the breaks are characterized by sharp 

 right angles, and in many places a stepladder arrangement is evident In the 

 early stages of attack the wood fibers turn red-brown and shrink. As a result, 

 fissures are formed in the walls of the tracheids, which extend diagonally across 

 the wall at an angle of approximately 45 degrees. (PI. XI, fig. i). The med- 

 ullary ray cells are at this point still intact, and hold together the more or less 

 brittle wood fibers. The next stage in the decomposition consists in the ab- 

 sorption of the medullary rays. This allows the wood fibers to contract more 

 than up to that time, and as a result breaks occur. These breaks form at first 

 so as to connect adjacent cavities left by the absorption of the medullary rays. 

 The wood fibers tend to curve in one direction or another and break at the 

 weakest point, namely, between two cavities, where the opportunity for curva- 

 ture is greatest. What determines the direction of curvature of the wood fibers 

 has not yet been explained. In the illustration the curvature is toward the 

 right. This curving has the effect of bringing medullary rays which are in 

 different longitudinal rows approximately into a line. Thus at " a " two cavi- 

 ties are shown which are separated by a curved fiber which sooner or later will 

 break, uniting the two. At first two ray cavities are joined, then more, until 

 long longitudinal holes are formed, such as are shown in fig. 4 of PI. XI. The 

 reason for the sharp edges is now very apparent, likewise why these fissure fig- 

 ures appear only on a tangential view, while on the radial view one simply sees 

 the fissures as lines extending at right angles across a ring of wood (PI. XIII). 



1 The plates referred to are also those of von Schrenk's bulletin. 



