ORGANIC SUBSTANCES AS PLANT FOOD 



237 



the external appearance is much like ordinary green plants; their 

 roots, however, are very weakly developed and end in haustoria, 

 by means of which they fasten themselves to the roots of other 

 plants. The physiology of these semiparasites has been studied in 

 detail lately by Heinricher, who was successful in growing them in 



Fig. 92. — An artificial culture of Melampyrum arvense on roots of the hazel (after 



Molisch) . 



artificial cultures (Fig. 92). It was found that their chlorophyll- 

 bearing organs work normally and that from the host plant they 

 receive only water and nutritive salts dissolved in it, in all prob- 

 ability, mainly nitrogenous substances. 



A somewhat higher degree of parasitism is exhibited by mistle- 

 toe (Viscum and Phoradendron, Fig. 93) and other representatives 

 of the family Loranthaceae. These plants inhabit the branches of 



