NOTE ON THE LARCH DISEASE. 39 



planting operations, it frequently happens that serious damage is 

 done to the protective covering. Unless, therefore, proper care is 

 exercised in these operations, the periderm is apt to get bruised 

 and torn, and the plants are thus rendered liable to attack. 



From the foregoing it is quite evident that there are only too 

 many agencies at work, opening up the way for the germinating 

 spores. When the germ-tube gains an entrance to the living 

 tissues, it very soon begins to form a mycelium, that is, a branch- 

 ing mass of the hyphee or threads which constitute the vegetative 

 part of the fungus. This mycelium spreads in the tissue of the 

 living bark, and may penetrate through the wood-body to the 

 pith. The living cells around the point of entrance are soon 

 killed, and, on drying up, form shrunken or depressed areas. The 

 tree endeavours to prevent a further spread of the parasite into 

 its tissues by surrounding it with a thick wall of cork, but this 

 barrier is of little use, since it can only be formed in the living 

 bark, and the fungus can therefore gain access to the living 

 tissues beyond through the wood. 



During the summer the fungus is held in check by the vital 

 activity of the living cells and the cork layer, but when the tree 

 has passed into the resting condition, and those cells have become 

 dormant, the spread of the mycelium is unhindered, and the 

 fungus is then able to increase the area of the blister. In this 

 way the struggle goes on between the host and parasite from year 

 to year. The cambium round the diseased area keeps on forming 

 new annual rings, so that, ultimately, the canker appears as a 

 flattening, or as a depression in the stem. The supply of food is 

 diverted to the side remote from the blister, with the result that 

 the cambium there being better nourished, forms broad year rings, 

 thus causing a projecting or swollen part on this side. The 

 tensions which are set up between the living and dead tissues 

 cause ruptures hei-e and there, which are followed by an outflow 

 of resin. 



If the tree is a vigorous one, and the other conditions not too 

 favourable for the fungus, the struggle between host and parasite 

 may go on for a great many years. Hartig records an example 

 in the Tyrol where the canker was a hundred years old. On the 

 other hand, if the growth of the tree is slow, and the other 

 conditions favourable to the parasite, the stem is very soon com- 

 pletely encircled by the canker and killed. 



The disease may be easily communicated from one tree to 



