.">() THE BOOK OP EVERGREENS. 



leaf. As all vegetable productions when in a perfectly 

 healthy state, are free from the numerous parasitic growths 

 that are common to diseased plants or trees, therefore, 

 when any appearance of the lower order of Cryptogamia, 

 such as fungi, mosses, and lichens, becomes visible, some- 

 thing must be assuredly wrong in the functions of the 

 plant itself. This parasitic vegetation is not the primary 

 cause of ill-health, as some imagine, and hence the mis- 

 chief is performed before these outward forms become ap- 

 parent ; and although these are charged with being the 

 prime instigators, they are in reality but the effect of dis- 

 ease previously contracted by the tree. 



The species that we have found to be most easily af- 

 fected, are the Pinus Austriaea, P. Laricio, P. Pyrenaica, 

 P. Pallasiana, with perhaps a few others ; and in every 

 case the diseased trees were members of the two-leaved 

 group of Pines. 



The disease known as Etiolation^ or blanching, entirely 

 destroys the verdure of plants, and renders them pale and 

 sickly. This arises from an insufficiency of light. It is 

 mostly observed on such plants as are growing in the dense 

 shade of trees ; but may occur from a variety of causes, 

 such as insects nestling in the rootlets and consum- 

 ing the food of the plant, thus debilitating the leaf so as 

 to render it insusceptible to the action of the light ; or the 

 same appearance may arise from extreme poverty of the 

 soil. It is not prevalent, and in the majority of instances 

 may be easily- detected and remedied. 



Gangrene, however, is of a different nature, and is 

 mostly confined to the half-hardy plants, or such as are 

 easily affected by sudden changes of the atmosphere. It 

 i- mainly attributable to two causes, the one arising 

 from an excessively high degree of temperature, the other, 

 from extreme cold. A very low temperature destroys or 

 shrivels the green leaves and shoots, turning them to a dark 



