NUTRITION 



3*3 



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 Ncottia: 

 by its stubby form and the frequent absence ^[jJ^l^SJ^ 1 ^ 

 of root hairs. In fact, the more the cases of phae. — After Magnus. (See 

 so-called mutualism are studied, the more it alsofi s s - IIo6 > iio 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 



