CHAP, ix Nutrition of Heterotrophic Plants 435 



fixation of atmospheric nitrogen in the soil has already 

 been alluded to. The association of a fungus and an alga 

 in a union of this kind to form the organism known as 

 a lichen was first definitely shown by Schwendener in 

 1860, though seven years previously Sachs had advanced 

 the idea of organic interdependence of what were then 

 thought the two tissue elements, the hyphae and the 

 gonidial cells, of the thallus of Collema. Reess put forward 

 identical views of the physiology of Collema in the same 

 year as Schwendener. The term symbiosis was applied to 

 the lichens by De Bary in 1879. 



Symbiosis in which the higher plants are concerned was 

 shown to be somewhat widespread. The connexion between 

 the roots of many plants growing near each other, which 

 was at first considered to indicate parasitism, was proved to 

 be symbiotic. The structure of the haustoria connecting the 

 Rhinantheae, the Santalaceae and other groups with their 

 so-called host-plants was investigated in 1875 by Solms- 

 Laubach, and the general morphological relationships were 

 studied by Koch in 1889 and 1891, and by Heinricher 

 from 1897 onwards. 



The tendency of saprophytism of this kind to develop 

 into parasitism was shown in the researches of Wiesner 

 (1871) and of Koch (1887) on Orobanche, of Koch (1880) and 

 Pierce (1894) on Cuscuta, and of Heinricher (1895) on 

 Lathraea. 



The relations of the Loranthaceae with their hosts were 

 studied by Pitra in 1861 ; he held that a transference of 

 water and inorganic salts from the host-plant was all that 

 takes place. A fuller symbiosis was suggested by Hartig 

 in 1875 and again by Bonnier in 1891. 



Symbiotic relations between fungi and the higher plants 

 were shown to be very widespread, the connexion existing 

 mainly in the roots. We have already discussed the pecu- 

 liarities of the root tubercles of the Leguminosae and other 



E 62 



