.9 YMBIOSIS. 



335 



plants which would furnish them similar external conditions 

 (fig. 372). 



LJ % 



A />' 



Fig. 372. Fig. 373. 



Fig. 372. — A portion of a filament of an alga (Ectocarpus^ showing at <( another alga 

 [Entoderma Wittrockii) which has embedded itself in the cell-wall. Magnified 480 

 diam. — After Wille. 

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

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

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

 Magnified ifiu diam.— After Kerner. 



459. (/') Mycorhiza. — 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 hyphae extending from the surface of the fungus 

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

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

 • onfined to certain definite portions of the roots, forming 

 upon them swellings from the si/.e 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 11)1011 the roots ^\ a 

 large number of herbaceous plants. The second form belongs 



