1 86 ANATOMY OF THE GYMNOSPERMS 



of soft brown mycelium, which often assumes very fantastic 

 shapes (69, IO). 



In the diffusion of a fungus through woody tissue two impor- 

 tant features may be noted : (i) the extent and direction of 

 development are determined in the first instance by the distribu- 

 tion of food materials ; (2) the distribution and progress of the 

 mycelium are independent of the presence of physical openings 

 in the structure. A few specific illustrations may serve to make 

 these statements clear. No matter what particular channels 

 may have permitted the mycelium to gain access to the interior, 

 its development appears to arise chiefly and first of all in the 

 medullary rays. The mycelium extends in the general direction 

 of the ray structure and therefore at right angles to the principal 

 lines of structure for the wood as a whole, filling the individual 

 cells with loosely felted masses of brown hyphae, from which are 

 developed short and variously divided branches. These latter 

 are particularly connected with the absorption of food substances, 

 and they are appropriately known as haustoria. It very fre- 

 quently happens that such growths of mycelium may be nearly 

 or altogether confined to the structure of the ray, the adjacent 

 tracheids being wholly devoid of them ; or there may be local 

 areas within which the mycelia extend vertically upward and 

 downward from the ray, invading the neighboring tracheids, 

 through which they extend for long distances. These facts 

 suggest that the medullary ray may offer more favorable condi- 

 tions for development than other parts of the structure, and 

 they make it desirable to examine its structural features some- 

 what more closely from this point of view. 



An examination of a medullary ray as exposed in radial sec- 

 tion shows that the upper, lower, and side walls -- particularly 

 the latter --are provided with definite pits, through which it 

 would be possible for the fungus to pass into adjacent cells with 

 very little opposition. But such favorable conditions are obvi- 

 ously not taken advantage of to any great extent, since neigh- 

 boring tracheids often remain quite free from the mycelium 

 while the ray cells are crowded with it. This would seem to 



