S Y MB 10 SIS. 



335 



plants which would furnish them similar external conditions 



(%• 372)- 



Fig. 372. Fig. 373. 



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



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



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



Magnified 480 diam.— After Kerner. 



459. (b) 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 

 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 



