14 A DISEASE OF THE WHITE ASH. 



The fungus under discussion is one of the most distinct forms of the 

 Fomes type of Polyporiis, and considering- the great variability of 

 form of many species of this genus it can be said to be remarkably 

 constant in most of its characters. 



MICROSCOPIC CHANGES IN THE AVOOD. 



The minute changes which the wood cells undergo are marked by 

 great distinctness and regularity. The wood of the ash forming the 

 bulk of the trunk serves as a repository for large quantities of starch. 

 Even in trees which are T5 to 100 years old one will find starch almost 

 at the center. In the ash the starch occurs in the form of small grains' 

 (PI. IV, fig. 6), filling the cells of the medullary rays and wood paren- 

 chyma. Fig. 1, PI. Ill, represents a cross section of wood (cut in 

 March), stained with iodine. The medullary rays appear almost as 

 black lines. 



One of th€ first changes noticeable in the wood when attacked by the 

 ash fungus is in connection with this starch. The region where the 

 starch changes is just outside of the dark line seen in PI. I. The large 

 grains (PI. IV, fig. 6) appear to break up into numerous smaller ones 

 (PI. IV, fig. T), and finally even these disappear. The change is a very 

 rapid one, and transition stages are very rare. No such regular 

 gradual dissolution of the grains occurs as is described by Hartig 

 as taking place in oak wood attacked by Polyporus suljyJiureus and 

 Polyj)orus igniarius. When stained with iodine one finds large grains 

 now and then, with channels through them (PI. IV, fig. 6), or more 

 frequently some which look as if the center had been dissolved out. In 

 several instances grains were found which stained brown with iodine 

 at the edges. This brown color then gradually passed in toward the 

 center of the grain. 



No hyphffi are present in the wood where the starch is breaking up. 

 This would indicate that a diastatic enzyme given off by the mycelium 

 precedes the latter for some distance. The first hyphaj are generally 

 several rings farther toward the middle of the trunk. The even extent 

 of the solution strengthens this supposition, for in a limited area of one 

 wood ring one and the same stage of dissolution is found at alwut 

 the same distance from the point where the fungus begins its growth. 

 After the disappearance of the smallest grains the cells formerly 

 filled with starch appear empty for several cell rows inward. Shortly 

 after the disappearance of the starch they become filled with a bright- 

 colored substance, which is probably liquid at first and hardens after 

 infiltration into the cells (PI. IV, fig. 2). This substance, which is 

 very soluble in alkalis, is probably some humus compound which 

 must be regarded as a decomposition product. It is distributed 

 throughout the medullary rays and the woody i^arenchyma, occupying 

 almost the identical cells which had harbored the starch. This will 



