NUTRITION 91 



nothing from association with the fungus, the parasite 

 gains everything from association with the green plant. 



Only about those remarkable structures called lichens 

 is there any difference of opinion. Lichens are composed 

 of algae and fungi living together. The algge — green, blue- 

 green, or brownish — contain chlorophyll, and by its aid man- 

 ufacture non-nitrogenous foods from carbon-dioxide and 

 water. Nitrates they absorb in solution. From these and 

 the sugars they elaborate amides and proteids like other 

 independent plants. The fungi, on the contrary, devoid of 

 chlorophyll, cannot elaborate non-nitrogenous foods and 

 must absorb them ready formed. In the lichen the fungus is 

 always closely applied to the algal cells, sends out short 

 branches which clasp the algal cells, and, in a considerable 

 number of already reported cases, these short branches send 

 still shorter haustoria into the algal cells.* Whether only 

 closely applied to the walls, or sending haustoria into the 

 cells, the fungus filaments are so placed that they can draw 

 food by osmosis from the alga. Because of the small size of 

 the alga, the always larger fungus cannot become entirely 

 enclosed within it ; on the contrary, the fungus surrounds the 

 alga with a more or less firm mycelium, confining the alga 

 between the parts of its body. The association of fungus 

 and alga, always intimate enough for the fungus to supply 

 itself osmotically with non-nitrogenous foods elaborated by 

 the alga, is in many cases so exhausting to the alga that 

 many of its cells become entirely emptied. In spite of this 

 evidence of the complete parasitism of the fungus, some 

 botanists claim that the alga benefits also. It is alleged 

 that the carbon-dioxide exhaled by the fungus, the mineral 

 salts dissolved and held in solution, the protection against 

 too rapid drying, too intense illumination, and too sudden 

 changes of temperature, are of sufficient value to the 

 alga to compensate it for the food taken from it and for 

 the deformities and limitations in its growth. It may be 



* Peirce, G. J. The nature of the association of alga and fungus in 

 lichens. Proc. Cal. Acad. Sci., Series III., Botany, vol. I., 1899. The rela- 

 tion of fungus and alga in lichens. American Naturalist, vol. XXXIV., 

 1900. 



