CONTINENTAL NOTES— FRANCE. 27 



up above the canker, or it may break off at that point, but 

 otherwise, though the wood at the actual canker is useless, 

 the damage, from the physiological point of view, is not great — 

 the growth of the tree is not much interfered with. The part 

 affected can be cut out, but since the canker may occur at any 

 point along the trunk, the length of the logs obtainable is 

 governed by its position, and this affects the price offered by 

 the merchants a good deal. 



In 1858 Mathieu discovered that the silver fir canker was 

 connected with the "witch's broom,' so often found on the 

 branches of this tree. The witch's broom, for all its appearance, 

 is not a parasite, but a distorted growth of the branch itself. 

 When the spore of the fungus Aecidium elatinum (rust) has 

 settled on the branch and caught on, the plant attempts to 

 fight the enemy by a precocious development. The branch 

 swells at this point, and the whole witch's broom becomes 

 infected, but after a time the latter falls off, leaving the 

 swelling, which eventually becomes a canker. 



But all attempts to propagate the fungus from one silver fir 

 to another failed until in 1901 Fischer, of Berne, discovered 

 that the aecidium spores on leaving the silver fir took to plants 

 of the Alsineae. Here the fungus is known as Melampsorella 

 caryophyllacearum. After this stage the silver fir is in- 

 fected. 



But the mycelium can only penetrate young shoots — four 

 years old at most. Hence the encircling canker has begun 

 on a leading shoot, while the partially encircling canker comes 

 from a side shoot. It is remarkable that the fungus does 

 not kill the shoot immediately, but only modifies its evolution. 

 The witch's broom is, as it were, a separate plant, which 

 eventually is killed (in fifteen years at most), separates, and falls 

 off. In the meantime it drops its leaves and re-shoots, in 

 the normal manner, but each year to find the new shoot 

 invaded by the mycelium. The fungus in the canker 

 dies eventually (within some forty years), but the distor- 

 tion of the stem remains always, in spite of its continued 

 growth. 



It is easy to reduce the damage from canker in silver fir 

 forests. One has but to cut off the branches on which are 

 witch's brooms. More than half are met with at below 7 metres 

 (some 23 feet) up the stem, because the Alsineae being ground 



