TISSUES AND SIMPLE ORGANS. 



145 



very probably indicates that they are partially dependent upon a 

 nitrogenous food-suj)ply. ' 



The following substances serve as food for bacteria and moulds : 

 the carbohydrates, various organic acids, glycerin, albuminous 



Fig. 85. — Haustoiium of Thesium pratense. 



(After Solms-Laubacli.) 



substances, peptone, leucin and asparagin. These substances and 

 many others Avere used by Pasteuk, and Nageli in numerous 

 culture-experiments. 



(c) SyTThbiosis. 



Externally symbiosis resembles parasitism in that it represents 

 the organic union uf one plant with another. On closer examina- 

 tion, however, we notice a marked difference. In the definition 

 of parasitism it was stated that the host-plant was in some way 

 injuriously affected. In symbiosis two plants live together as in 

 parasitism, but they mutually assist each other in their life func- 

 tions, especially in nutrition. The term symbiosis was introduced 

 by DE Bary in his work entitled " Die Erscheinung der Sym- 

 biose," published m 1879. 



The most important example is met with in lichens (Figs. 86- 

 88), the true nature of which was made known by Schwendener's 

 epoch-making researches (1 860-1 8T0). Other important researches 

 in the same line were carried on by Bornet, de Bary, Stahl, 



' Pfeffek, Pflanzenpbysiologie. 



