THE FACTORS OF AN' EPIDEMIC. 153 



gone, and the tree passes forthwith into its 

 summer state. 



In the plains, on the contrary, the Larch begins 

 to open at varying dates from March to May, and 

 during the tardy spring encounters all kinds of 

 vicissitudes in the way of frosts and cold winds 

 following on warm days which have started the 

 root-action for we must bear in mind that the 

 roots are more easily awakened after our warmer 

 winters than is safe for the tree. 



It amounts to this, therefore, that in the plains 

 the long continued period of foliation allows in- 

 sects, frost, winds, etc., some six weeks or two 

 months in which to injure the slowly sprouting 

 tender shoots, whereas in the mountain heights 

 they have only a fortnight or so in which to do 

 such damage. That the lower altitude and longer 

 summer are not in themselves inimical to Larch 

 is proved by the splendid growths made by the 

 trees first planted a century ago. Then came 

 the epidemic of Larch-disease : the fungus, which 

 is merely endemic i.e. obtains a livelihood here 

 and there on odd trees, or groups of trees in 

 warmer or damper nooks in the Alps, was 

 favoured by the more numerous points of attack 

 afforded to its spores by injuries due to insects 

 Coleophora, Chermes, etc, and frost wounds, as 

 well as by the longer periods of moist dull 

 weather, and the longer season of foliation. More- 

 over, as time went on almost every consignment 

 of young Larch-trees sent abroad was already 

 infected. Here again, then, we find the factors 



