202 VEGETABLE PHYSIOLOGY 
One of the best known cases of symbiosis in the strict 
sense is that of the Lichens. These are lowly organisms 
which are epiphytic upon tree-trunks, old walls, rocks, and 
other supporting structures. They are composed always of 
two distinct plants, an Alga and a Fungus, which are closely 
united together to form a kind of thallus (fig. 97). The 
relative modes of arrangement differ in different species, 
and many alge and many fungi are found to be capable of 
entering into such an association. The advantages which 
result to the two constituents of the lichen are consider- 
able. The alga, which possesses chlorophyll, is able to con- 
struct. carbohydrate materials by its instrumentality, and 
after their formation these are shared by the fungus, 
which has no such construc- 
ASSSee=Aroes tive powers. The fungus is 
BOLUS ear eo LY, able to condense aqueous 
SESS USOC EIA ee 
oe SEO) vapour, which is very neces- 
sary in the dry situations 
lichens occupy. It can thus 
dissolve much of the dust 
and other débris of its rest- 
ing place, and so carry raw 
material to the constructive 
algal cells. It also attaches 
the thallus to the substratum. 
Both partners can no doubt 
take part in the construc- 
Np tion of proteins. The rela- 
Fra. 97.—Szerton or a Trcusn tionship affords a further 
Maene Araan Ceuts (9) IN 78 advantage, for the compound 
Hypita (m). (After Sachs.) organism is much better able 
than either of its separate 
constituents to resist adverse conditions of temperature, 
drought, &e. 
A similar symbiosis is met with in the so-called kephir 
organism and others of the same kind. In these cases the 
two constituents are a yeast and a bacterium, the former of 
