FUNGOUS AND PHYSIOLOGICAL DISEASES. 



485 



the alga, which, in return for these good offices, supplies 

 the former with the result of its assimilative processes. 



The moist, warm climate of some of the citrus dis- 

 tricts is ideal for their development. In many orange 

 groves the trunks and the larger branches of the trees are 

 covered with lichens; they are obnoxious to the eye, and 

 to a certain degree detrimental to the health of the tree. 

 They are not parasitic, but they prevent a free inter- 

 change of gases through the bark, thus to some extent, 

 producing a condition injurious to the health of the tree; 

 and certain it is, they are never so plentiful on healthy, 

 vigorous trees as upon those which are in poor condition. 

 They serve also as a harborage for insects and insect eggs. 



Consequently, it is very desirable 

 that the trunks of the trees should 

 be kept free from lichens, and, for- 

 tunately, this is easily accomplished 

 at a very slight cost. One treatment 

 every two or three years will be suf- 

 ficient to keep the trunks in a clean, 

 healthy condition. 



Besides those found on the trunk 

 and large branches, a lichen, Strig- 

 ula complanata (Fee and Mont.) 

 Nyl., is frequently met with on the 

 leaves of citrus trees. It occurs in 

 the form of small ashy-gray dots and 

 blotches on the upper surface of the 

 leaves, Fig. 106. Here and there on 

 these blotches may be seen small, 

 black specks, the fruiting bodies of 

 . ice. Lichen (striguia tne lichen. This species injuriously 

 affects the leaves b y preventing the 



