NUTRITION 
383 
FrG. 654. Endotrophic 
making longer because the encompassing fungus by its spongy tex- 
ture retains rainwater longer than would the algae alone. Mycorhiza 
is another instance of so-called mutualism, in which fungi associate 
themselves with the roots of certain plants, 
especially the oaks (Cupuliferae), the heaths 
(Ericaceae), and the orchids (Orchidaceae). 
Sometimes they jacket the rootlets with a 
weft of filaments (ectotrophic mycorhiza, fig. 
653), and sometimes they penetrate the corti- 
cal cells, forming a tangle about the nucleus 
(endotrophic mycorhiza, fig. 654). The fungi 
are supposed to aid the root in acquiring 
water and food materials (especially nitrogen 
compounds, which they themselves may form 
from the free nitrogen of the air) from the 
soil. Certainly they derive some food from 
the root, and injury to the root is suggested mycorhiza of Neottia: a, host 
. ^ it / 1,1 < cell with active fungus hyphae: 
by its stubby form and the frequent absence b> cell with deg enerating hy- 
of root hairs. In fact, the more the cases of phae. After MAGNUS. (See 
so-called mutualism are studied, the more it a lgs ' II0 ' ZI 7 '' 
becomes evident that they are only cases of modified parasitism, with 
minor injury to the host. (See Part III on reciprocal parasitism.) 
Injury by parasites. On the other hand, the drain on the food re- 
sources of the host may be severe, so weakening it that it succumbs to 
adverse conditions which otherwise could be overcome. Quite apart 
from this weakening for lack of food, the parasite may act as a stimulus 
to local growth, or it may produce injurious substances which cause local 
or even general death. The location of a parasite is often marked by 
deformities; leaves are crinkled or thickened, as in peach curl; circum- 
scribed swellings of peculiar and fantastic or beautiful forms (galls) grow 
on leaves or stems (fig. 655); even large tumors are formed, as in the 
black knot of cherry and plum trees. Local death is another common 
mark of the presence of a parasite. The fire blight of apple and pear 
trees, due to parasitic bacteria, gets its name because young shoots 
are killed for a distance of 20 to 50 cm., and the withered brown leaves 
make the tree look as though it had been scorched by a fire. General 
death in large plants is seldom produced by a parasite unless it inter- 
feres with the water supply or invades the entire organism. In wilt 
disease the parasite blocks the tracheae, interfering with the supply of 
