DISPOSITION OF PLANTS TO DISEASE. 61 



situations favour reproduction of mildew and other diseases ; 

 under such conditions a rapid increase of potato-disease during 

 July is easily observable and may be safely foretold. 



The extension of Herpotrichia is greatly facilitated by snow, 

 which weighs down young plants or branches of spruce and 

 pins them to the soil, where the fungus develops on its host 

 under the snow-covering. On this account elevated situations 

 and hole-planting render the spruce liable to disease. 



Many plants which, as a rule, suffer from fungus-diseases 

 will be found to. remain exempt in open or dry situations, or 

 during a dry period. The tops of trees are not attacked by 

 many fungi which frequent the lower parts of the crown. This 

 is particularly the case with epiphytic lichens and certain fungi, 

 which require a high degree of air-moisture. Trichosphaeria 

 parasitica, always very abundant in damp silver fir regenerations, 

 is almost absent from free-standing trees, or from the higher 

 parts of the crown in closed forest. It is, in fact, a parasite 

 well adapted for extension in the crowded masses natural to 

 the early growth of the fir, and the host is, during its youth, 

 disposed to disease from this particular parasite. A fungus on 

 the beech behaves similarly, occurring in Bavaria only in the 

 very damp parts of close high forest and in Alpine gorges. 

 Other fungi have better means of protection against drought, 

 for example, Hysterium macrosporium has its spores enclosed in 

 gelatinous envelopes and may be found on the highest point of 

 the spruce, although, on the whole, its distribution is most 

 favoured by moisture. Fungi which frequent algae, or are dis- 

 tributed by means of zoospores, depend absolutely on moisture ; 

 hence they frequent hosts growing on banks of streams, places 

 liable to flooding, or low-lying moist meadows, whereas the same 

 host-species remains completely exempt from their attacks in a 

 dry locality. 



A plant may be said to be in a condition of abnormal 

 disposition to disease when deprived of its natural protection. 

 Thus wounds of any kind render a plant disposed to infection 

 from wound-parasites, which are unable to harm uninjured parts. 

 After severe hail-storms an outbreak of Nectria ditissima is not 

 unfrequent amongst regenerated beech, or even in the canopy 

 of older forest. I have also observed an extensive outbreak of 

 Cucurbitaria laburni on laburnum near Munich, obviously due 



