TISSUES AND SIMPLE ORGANS. 



145 



very probably indicates that they are partially dependent upon a 

 nitrogenous food-supply. 1 



The following substances serve as food for bacteria and moulds : 

 the carbohydrates, various organic acids, glycerin, albuminous. 



FIG. 85. Haustorium of TJiesium pratense. 

 (After Solms-Laubach.) 



substances, peptone, leucin and asparagin. These substances and 

 many others were used by PASTEUR, and NAGELI in numerous 

 culture-experiments. 



(<?) Symbiosis. 



Externally symbiosis resembles parasitism in that it represents 

 the organic union of 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 in 1879. 



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

 88), the true nature of which was made known by SCHWENDENER'S 

 epoch-making researches (1860-1870). Other important researches 

 in the same line were carried on by BORNET, DE BARY, STAHL> 



1 PPEFFER, Pflanzenphysiologie. 



