32 BULLETIN 1380, IT. S. DEPARTMENT OF AGRICULTURE 
According to the general conception of lichens, the following ex- 
planation may be given. It is assumed that the fungous and algal 
elements live together in mutual cooperation. The latter through 
assimilation produces organic and inorganic substances from which 
the former in part obtains nourishment. The fungus in turn sup- 
plies certain mineral salts from the substratum and protects the 
algal plant from drought, which enables it to live on such dry sur- 
faces as the bark of trees. Such cooperation between plants is 
known as symbiosis and is analogous in part to the relation of mistle- 
toes and their hosts. In the latter, however, the relation, instead of 
being mutualistic, is only a temporary condition, since the host is 
finally destroyed. 
The lichens are rarely parasitic. They live as epiphytes on the 
dead bark and derive little or no nourishment from it. Through 
the secretion of certain acids the outer layers of the epidermis may 
be destroyed, but the vital tissues beneath are not affected. The 
excessive development of lie-hens may cause damage by depriving 
green surfaces of light and air. Their presence in quantity may 
in some cases be indicative of the vigor of the tree on which they 
are found. When the growth of a tree has become so retarded that 
there is little or no expansion or exfoliation of the bark, lichens 
develop in great numbers. This condition was observed on Hevea 
growing in the compact white clay soils of swampy areas. On high 
well-drained red or yellow soils lichens on Hevea were much less con- 
spicuous. The development of a small black lichen (Pyrgillua sp.) 
on Hevea growing in wet poor soil was frequently conspicuous. It 
developed at the ground line and extended up the trunk for a dis- 
tance of 2 or 3 feet. The appearance was that of a black powdery 
fungus enveloping the roots and lower trunk. The bark over the 
area was scaly and could be readily scraped off. In very wet situa- 
tions in dense shade this lichen completely enveloped the trunks of 
small trees and occasionally extended to the lowest branches. This 
may have been the organism mentioned by Akers, who states that 
"the most common form of bark disease is a fungoid growth carry- 
ing a black powdery substance on the surface. It appears first 
near the foot of the tree and gradually spreads up the stem to the 
main lateral branches. " He further states that ww for the most part 
it is found in low-lying localities where the soil is a stiff yellow 
clay." Although he suggests the remedy of adequate drainage and 
the application of lime, no information is given as to the identity 
of the organism. 
Some of the most common incrusting lichens on the bark of Hevea 
are Vhaeographina scalpturata, Graphis afzelii, Phaeographina sp., 
Trypethelium catervarium, Sarcographia labyrinthica, and Pyrgil- 
lus sp. 
The common foliose species were Cora pavonia and Chiodecton 
sanguin&um. 
The excessive growth of lichens may be prevented by spraying 
the stems with a copper-sulphate solution. This solution is injurious 
to leaves and should be confined to the main stem and branches. 
The green flat ribbonlike dichotomously branched growths occa- 
sionally seen on the bark of Hevea in wet places are liverworts and 
are related to the mosses. They are epiphytic and are of no conse- 
quence. 
