366 THE FOOD OF PLANTS 



ment impossible, and apparently the seeds of Orobanche and Lathraca 

 germinate only when contact with the root of a host-plant exercises a cer- 

 tain stimulus upon them which is probably chemical in nature. A special 

 chemical stimulus seems also to be necessary for the development of the 

 spores of certain fungi l . Similarly, a parasite may be unable to develop 

 in a nutrient solution because of the non-performance of certain functions 

 exercised when it penetrates a host, and the difficulty of cultivating many 

 motile organisms upon solid nutrient media may be due to the fact that 

 their inability to move exercises a certain injurious effect upon them 2 . As 

 a matter of fact the enforced cessation of the assimilation of carbon dioxide 

 induces a pathological condition in green leaves, while in Ciiscnta a con- 

 tact-stimulus causes a localized activity of growth leading to the formation 

 of haustoria. 



Precise data on these questions have not as yet been obtained, nor 

 has it been determined why Euphrasia, Rhinanthns, Thcsium, &c. can only 

 grow when they are able to form haustorial connexions with the roots 

 of other plants, for their own roots obtain a sufficiency of water and salts 

 from the soil, and their chlorophyll-apparatus provides an abundant supply 

 of carbohydrates. It is possible that such plants must obtain nitrogenous 

 compounds or certain constituents of the ash parasitically 3 , and as a matter 

 of fact the growth of carnivorous plants is favoured by a supply of 

 nitrogenous organic substances, while other plants are unable to live 

 unless supplied with protcid. Examples of these so-called ' peptone- 

 organisms ' arc afforded by many fungi and bacteria, while the same 

 peculiarity is exhibited by certain algae, and especially those which may 

 take part in the formation of lichens 4 . In return for the carbohydrate 

 supplies received, the fungal components of many lichens seem to transfer 

 a poition of the synthesized proteids to the algse, and the former have 

 become so markedly adapted to this special mode of existence that they 

 never occur free in nature, and can be cultivated only with difficulty 

 when isolated r \ Hence it is evident that we are dealing with a special 



1 Koch, Entwick. d. Orobanchen, 1887, p. 3. Uhinantlms seeds germinate without any such 

 stimulus. Koch, Jahrb. f. wiss. Hot., 1891, Bd. xxil, p. 4. Lalhraca: Heinricher, Her. d. Bot. 

 Ges., Generalvers., 1894, p. 126 ; Jahrb. f. wiss. Bot., 1897, Bd. xxxi, p. 79, footnote. Fungi : cf. 

 de Bary, Fungi, Mycetozoa and Bacteria, 1894, p. 376. According to Benecke (Jahrb. f. wiss. Bot., 

 1895, Bd. xxvill, p. 501), the spores of Aspergillus niger will not germinate in pure water. 



* An entirely fluid diet ultimately exercises an injurious effect upon man. Many motile 

 organisms may be successfully cultivated on solid nutrient media. 



8 [\Vhen closely sown, the seedlings may be parasitic upon one another (Heinricher, 1. c., 1897, 

 p. 87 ; Wettstein, Oesterr. Bot. Zeitschr., 1897, No. 9, p. 323), a few plants surviving and reaching 

 maturity. The seeds germinate v Rhinantheae) without contact with a host-plant.] 



* Beyerinck, Bot. Zeitung, 1890, pp. 730, 746, 766. According to Klebs (Die Beclingungen d. 

 Fortpflanzung, &c., 1896), Scenedesmus does not require peptone, but peptone-organisms may occur 

 more commonly among the Oscillariae, Euglenae, &c. 



5 A. M oiler, Die Cultur flechtenbildender Ascomyceten, Miinster, 1887. 



