SYMBIOSIS. 



335 



plants which would furnish them similar external conditions 

 (fig- 372). 



Fig. 372. FiG. 373. 



Fig. 372. — A portion of a filament of an a\g?i { Fctocar/'us^ showing at « anotlier alga 



{EntL'th'j-ina li'rttrockii) which has embedded itself in the cell-wall. I\Iagnilied 480 



diam. — After "Wille. 

 Fig. 373. — A, a tuft of rootlets of white poplar forming mycorhiza. Natural size. B, 



a portion of a transverse section of one of these rootlets, showing the mantle of fungus 



mycelium and the growth of the hyph^ also in some of the outer cells of the root. 



Magnified 480 diam, — After Kerner. 



459. (b) MycorMza. — Mutualism between the roots of the 

 seed plants and certain fungi is common. Such a combina- 

 tion of root and fungus is called a mycorhiza. The fungus 

 forms a jacket over the outside of the root (figs. 373, 374), 

 taking the place and work of the root hairs by means of 

 strands of hypha; extending from the surface of the fungus 

 jacket (fig. 374) ; or it grows inside the cells of the cortex 

 and epidermis, forming knotted masses (fig. 375 ) ; or it is 

 confined to certain definite portions of the roots, forming 

 upon them swellings from the size of a hazelnut to the size 

 of a man's head. The first form is especially common upon 

 the roots of the oak, elm, walnut, apple, pear, maple, ash, and 

 related trees. It has also been found upon the roots of a 

 large number of herbaceous plants. The second form belongs 



