52 DISEASES OF TROPICAL PLANTS on. 



Order 9. LICHENS. The lichens are probably the 

 most remarkable plants known, since each plant is 

 really two plants (an alga and a fungus), which 

 have united, forming a sort of unequal partnership 

 (symbiosis). The hyphae of the fungus are interwoven 

 among and around the cells of the alga. The alga, 

 because of its chlorophyll, is able to form the organic 

 foods which are partly absorbed by the fungus. The 

 vegetative portion of the fungus holds moisture, and 

 secures the necessary mineral food. However, it appears 





. i 







FIG. 32. Melancoimuu state of Tricosphaeria sacchari. 

 (After Cook and Home, Bui. 7, Estacitin Central Agronomica de Cuba.} 



that the partnership is not equal, but that the fungus 

 receives more from the alga than it gives in return. 

 The lichens grow and thrive only during seasons of 

 considerable moisture. When dry they become inactive, 

 and remain in a resting condition until the conditions 

 are again favourable for growth. They are sometimes 

 quite troublesome, especially in the cacao groves. 



FUNGI IMPERFECTI 



Under this term we group a great number of poorly 

 understood species. They are analogous to the conidial 

 stages of the Ascomycetes, and some of them are doubt- 

 less the conidial forms of true Ascomycetes ; but the 



