INJURIES INDUCED BY PLANTS 189 



cell-wall into cellulose, a change which begins on the side next 

 the lumen, and advances outwards. The cellulose is soon 

 completely dissolved, and at last the delicate skeleton of the 

 middle lamellae also disappears. At certain points this process 

 proceeds with great rapidity. Here and there, for instance, one 

 finds that the tracheides, in immediate proximity to the medullary 

 rays, are filled with a brown fluid, which has probably originated 

 in the latter, and which discolours and nourishes the mycelium 

 to such an extent that a brown " mycelial nest " is formed. So 

 energetic is the action of the ferment in the neighbour- 

 hood of these bodies that the encrusting substances entirely 

 disappear from the adjoining tracheides, which, to the dis- 

 tance of several millimeters, are completely transformed into 

 cellulose, and thus become colourless in other words, white. 

 Almost immediately after being converted into cellulose the 

 middle lamella disappears entirely, and the individual elements 

 of the wood thus become isolated, so that, when disturbed by a 

 needle, they fall apart like the strands of asbestos. Gradually 

 they are dissolved, and holes, which are constantly increasing in 

 size, are formed in the crumbly wood. 



While the mycelium thus decomposes the wood, sometimes to 

 a height exceeding eight yards, the parasite advances much 

 more slowly in the cortex, where its presence is betrayed by three 

 distinct phenomena. From the point of infection the mycelium 

 spreads both towards the root-apices and towards the stem. It 

 kills the cortex, and consequently the root, and when, after some 

 years, it has reached the stem, it spreads from the stool on to 

 roots that have hitherto remained sound. As soon as these are 

 also attacked by the disease, the tree dies. 



A second function of the mycelium that grows in the cortex 

 consists in the formation of sporophores, which appear here' and 

 there between the bark-scales of the roots or stool. These lead 

 to the production of fresh seats of disease in the plantation, as 

 has been already described. 



A third function is concerned with the spread of the disease 

 subterraneously owing to infection by the mycelium. Where a 

 diseased ro6t comes into contact with the sound root of an 

 adjoining tree (Fig. 121), or where the two are positively grown 

 (grafted) together as may very frequently be observed in a dense 



