PHYSIOLOGY 



231 



with other individuals of the same species. Thesium, belonging to the 

 Santalaceae, and the following genera of the Ebinanthaceae, Rhinan- 

 thits, Euphrasia, Pedicularis, Bartsia, and Tozzia may be mentioned as 

 examples of plants showing these peculiar conditions. In Tozzia the 

 parasitism is well marked in the earliest developmental stages. The 

 Mistletoe (Viscum album), although strictly parasitic, possesses, never- 

 theless, like many of the allied foreign genera of the Loranthaceae, 

 fairly large leaves well supplied with chloro- 

 phyll, and fully able to provide all the 

 carbohydrates required. It obtains, how- 

 ever, from the host plant (as HEINRICHER 

 has also shown to be probable in the case 

 of the Ehinanthaceae) its supply of water 

 and dissolved salts. Another member of the 

 Rhinanthaceae, Melampyrum, though prim- 

 arily a root parasite, has, on the other hand, 

 adopted a saprophytic mode of life ( 49 ). 



Humus plants, like some of the Orchi- 

 daceae (Neottia, Coralliorrhiza, etc.), and the 

 Monotropeae, are restricted to a purely 

 saprophytic mode of nutrition, and to that 

 end utilise the leaf -mould accumulated 

 under trees. 



The roots and rhizomes of these saprophytes stand 

 in most intimate relation with fungal hyplise. Tlie 

 same indeed holds for the majority of green plants 

 which grow in woods and heaths, where the soil is 

 rich in humus. The fungal hyphse are sometimes 

 present in coiled masses within the cells of a definite 

 zone of the cortex, only occasional filaments passing 

 outwards to the soil. In other cases the fungus 

 surrounds the young roots with a dense investment 

 of interwoven hyphae. The former arrangement i* 

 spoken of as E~NDOTROPHIC, the latter as EXOTROPHIC 

 MYCOKHIZA ; the two types are, however, connected 

 by intermediate forms. A direct exchange of sub- 

 stance between the soil and the root would appeal- 

 impossible in the case of the exotrophic mycorhiza. 

 On this ground, and because, despite the reduced surface exposed to the humus- 

 containing soil by the roots or rhizomes of total saprophytes, these obtain sufficient 

 nutriment, a co-operation of the fungus in the nutrition of such plants has 

 been assumed. The results of culture experiments lend further support to this 

 view. Little is known, however, as to the nature of the relation between the fungus 

 and the saprophyte. The view of PERCY GROOM and JANSE that in the case of endo- 

 trophic jnycorhizas the fungus supplies combined nitrogen, i.e. is ultimately digested 

 by thejoot, is supported by the work of W. MAGNUS, SHIBATA, and HILTNER. STAHL 

 on the other hand sees its use in a better supply of salts from the soil. The latter 

 view, however, could only be a sufficient explanation in the case of green plants ( so ). 



FIG. 204. A root of Vicia Fvba, 

 with numerous root -tubercles. 

 (Reduced.) 



