42 



characterized by sharp right angles, and in many places a stepladder 

 arrangement is evident. In the early stages of attack the wood libers 

 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. 1). The medullary ray 

 cells are at this period still intact, and hold together the more or less 

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

 absorption of the medullary rays. This allows the wood fibers to con- 

 tract 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 absorp- 

 tion of the medullary rays. The wood fibers tend to curve in one direc- 

 tion or another and break at the weakest point, namely, between two 

 cavities, where the opportunity for curvature is greatest. What deter- 

 mines 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 efl'ect of bringing medullary rays which are in difler- 

 ent longitudinal rows approximately into a line. Thus at '^ a--' two cav- 

 ities 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. -I of PI. XL The reason for the sharp angles is now very 

 apparent, likewise why these fissure figures appear only on a tangen- 

 tial view while on the radial view one simply sees the fissures as lines 

 extending at right angles across a ring of wood (PI. XIU). 



The marking of the individual wood cells is a very regular one. 

 The fissures extend throug-h the secondary lamella, and at first sio-ht 

 remind one of those which the mycelium of Polyporus schiceinitzii 

 induces. The latter are very much steeper, however, and do not occur 

 at such frequent intervals. 



The mycelium of Polyporus sidfureus is colorless and is present 

 only here and there in the wood cells, a fact to which Hartig calls 

 attention. No spores, such as are so common when this fungus grows 

 in Oak wood, were seen in the Spruce wood, although diligent search 

 was made for them. 



FRUITING ORGAN. 



The sporophores of Polyporus sidfureus are among the commonest 

 and best known of the largest fungi. The sulphur-yellow shelves of 

 this fungus occur widely distributed throughout the United States, 

 and are found in late August and September on many of the Oaks, 

 Walnut, and other broad-leaf trees. A large number of sporophores 

 usually appear together, one above the other, when growing from 

 an upright trunk, or scattered here or there on a prostrate log. 

 They grow on living trees and on the dead trunks also, for several 

 years after the latter have fallen. A marked periodicity in this respect 



