392 PROTF,CriON ArjATNST PLANTS. 



/?. Parasites, such as dodder (X'lisruta), germinating on the 



ground, l)ut eventually attaching themselves to forest 



plants. 



y. Parasites nourished partly hy their own roots, and partly 



by attaching themselves to useful plants. Broomrape 



{Orohanche) on broom, furze, ivy, hemp, lucerne, etc. 



Lathraca squamaria, L., on the roots of trees, especially 



hazel. 



xi. Epiplii/trs. Lichens do not remove sap from forest 



plants, but live on water, air, and rotting baric, but when 



attached to trees they are injurious by blocking the lenticels 



and preventing necessary aeration of the internal tissues. 



They harbour numerous insects, and increase the weight of 



snow on the crowns of trees during winter. Trees with rough 



bark, such as spruce, larch and pines, are most subject to 



attacks of lichens, which increase in number with the age of 



their hosts. The l)ranches are more subject than the boles to 



these attacks, 



Incrustating lichens, such as Imhricaria, Ilatt/sma, and 

 Lecanora are worse than beard-like lichens, Usitia, Ercniia, 

 and Alectoria. Lichens do most damage in moist valleys and 

 in crowded woods, 



xii. Some weeds serve as hosts to injurious fungi, which 

 may afterwards spread to forest trees or to agricultural crops. 

 Many i^arasitic fungi are most frequently found on weeds, 

 or attack agricultural crops only after passing one stage 

 of their existence on a weed. 



Thus, certain grasses — e.g., Arrhenathcntm and Arena — 

 spread certain kinds of rusts on to cereal crops, such as 

 Tilletia caries, Tub, and Ustila[/o carho, Tub Puccinia f/ra- 

 ininis, Pers., on wheat comes from Accidiiim Bcrheridis on the 

 barberry ; and oat-rust {P. coronata, Corda) from Ijuckthorn, 

 Foresters should always look with suspicion on any fungus 

 appearing on wild plants. 



B. Special Forms of Damage done by "Weeds. 



The amount of damage done to forests by particular weeds 

 depends on their wide dissemination and on the vigour and 

 special nature of their growth. These are determined chiefly 



