196 VEGETABLE PHYSIOLOGY 
sitism. Others speak of reciprocal and antagonistic sym- 
biosis, to indicate these two different kinds of association. 
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- 
—.. 
SACRA ti The f 
os (Codes Oe f ‘tive powers. e fungus is 
ee wineng able to condense aqueous 
Fes ed ge B)Se Megs vapour, which is very neces- 
“amen eee ese ® sary in the dry situations 
DS SLES Spare ee 
2. LS ISSIEAD m 
Fic. 97.—Sgection or a LicHEN sHow- 
ING ALGAL CELLS (g) IN THE MIDST 
OF A NETWORK OF FUNGAL HYPHE 
(m), (After Sachs.) 
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 construe- 
tion of proteins. The rela- 
tionship affords a further 
advantage, for the compound 
organism is much _ better 
able than either of its separate constituents to resist 
adverse conditions of temperature, drought, Sc. 
A similar symbiosis is met with in the so-called kephir 
organism and others of the same kind. 
In these cases the 
