SPECIAL ADAPTATIONS 369 



Thus Euphrasia, Rhinanthus, Melampyrum arvense, &c., seem to be obligate root- 

 parasites, whereas according to Regel, Pedicularis and Bartsia, and according to 

 Kerner, Odontites lutea, can grow even when unable to form haustoria on the roots 

 of other plants 1 . Thesium seems also to be an obligate root-parasite, but it is 

 not yet certain whether the same is the case with the indigenous species of 

 PoZygala"*. Melampyrum pratense appears, however, to be a saprophyte, for 

 according to Koch it develops haustoria only in connexion with dead parts of 

 other plants 3 . 



On the other hand, in Orobanche, Cuscuta, Rafflesia, and many other parasites, 

 the vascular tissues of the host and parasite enter into intimate union similar to 

 that taking place between a graft and graft-stock 4 . A close connexion of this kind 

 is not always attained v&Euphrasia or Rhinanthus 5 , and in Viscum only the tracheae 

 of the wood unite, according to Peirce, indicating that Viscum obtains only water 

 and salts from its host. These anatomical facts do not, however, form conclusive 

 proof, for both nitrogenous and non-nitrogenous organic food-materials may be 

 transferred in abundance and with considerable rapidity through parenchymatous 

 tissues. Even Pitra's 6 experiments, made by ringing the infested branches, do not 

 afford decisive evidence that Viscum obtains only water and mineral constituents 

 from the host-plant. It has already been mentioned that partial mutualism may 

 exist between the mistletoe and its host-plant, an idea first put forward by 

 R. Hartig (Bot. Jahresb., 1875, p. 955). 



SECTION 65. Special Adaptations. 



A plant can develop in a particular locality only when its structure is 

 such that under the existing conditions it is able to obtain a sufficient 

 supply of organic food. Hence the shape of autotrophic plants is almost 

 entirely determined by the necessity of assigning a suitable position to the 

 chlorophyll-apparatus, whereas in non-chlorophyllous plants which obtain 

 their organic food from widely different sources, it is only to be expected 

 that many remarkable peculiarities in their form and mode of life should 

 have been developed. It must, however, suffice to give a general account of 

 the special adaptions, or reciprocal combinations by which the end in view 

 is reached. 



It must be remembered that with the exception of Myxomycetes 



1 The parasitism was first discovered by Decaisne, Ann. d. sci. nat., 1847, iii. st'r., T. vni, p. 2. 

 Cf. Koch, Jahrb. f. wiss. Bot., 1891, Bd. xxn, p. i. [On the autoparasitism of Eufhrasia, &c., see 

 Heinricher, 1. c., and Wettstein, 1. c.] Regel, Die Schmarotzergewachse, Zurich, 1854, p. 34 ; Kemer, 

 Pflanzenleben, 1887, Bd. I, p. 167. [Cf. also Heinricher, Die griinen Halbschmarotzer, Jahrb. f. 

 wiss. Bot., 1898, Bd. xxxn, p. 309.] 



a Ficke, Abhandlungen d. naturwiss. Vereins zu Bremen, 1875. Bd. iv, p. 378. 



3 Koch, Jahrb. f. wiss. Bot., 1889, Bd. xx, p. 33. 



4 Koch, Entwickelung von Orobanche, 1887, p. 63. Cuscuta: Peirce, Annals of Botany, 1893, 



Bd. VII, p. 324. 



5 Koch, Jahrb. f. wiss. Bot., Bd. xx, p. 22 ; Bd. xxn, p. 18. 



6 Pitra, Bot. Zeitung, 1861, p. 63. 



PFEFFER B D 



