SPECIAL ADAPTATIONS 379 



Fr. Darwin observed that plants of Drosera rotundifolia fed with meat 

 grew more strongly and produced heavier and more numerous seeds ; while 

 Biisgen obtained still more marked differences by commencing his com- 

 parative experiments with seedlings and feeding a portion of them for two 

 years with aphides x . 



In the pitchers of Sarracenia, Darlingtonia, Ccphalotus, and probably 

 also in the bladders of Utricnlaria, no enzyme is excreted, but nevertheless 

 soluble nitrogenous products set free by bacterial decomposition may be 

 absorbed. Small captured Crustacea often remain living in the bladders of 

 Utricnlaria for a long time, and it is possible that a reciprocal relationship 

 exists between the animal and the plant, the excreta of the former being 

 utilized by the latter 2 . 



Tischutkin 3 erroneously states that only bacterial digestion occurs in car- 

 nivorous plants, for in Drosera, Drosophyllum, Dionaea, Nepenthes, &c., there can 

 be no doubt as to the presence of a proteolytic enzyme. This appears to be 

 a pepsin active only in an acid solution, but the nature of the acid is unknown. 

 De Wevre has recently shown that Drosophyllum produces no formic acid 4 , nor has 

 the presence of this acid been demonstrated with certainty in Dionaea and Drosera*. 

 Further, it is uncertain whether malic and citric acids are present in the pitcher 

 fluid of Nepenthes, either free or in the form of salts 6 . The enzyme and the 

 acid are either excreted continuously or only after stimulation, and in the latter 

 case nitrogenous and other chemical substances act as stimuli, while mere 

 mechanical irritation may cause an increased excretion. In Dionaea a marked 

 excretion of both acid and enzyme follows stimulation : the ferment is always 

 present in the pitcher-fluid of Nepenthes, which becomes acid however only after 

 the application of a chemical stimulus 7 . 



Although the fact that certain plants captured insects was known in the 



1 Fr. Darwin, Exp. on the nutrition of Drosera, 1878, Linn. Soc. Jonrn., Vol. XVII ; also Keller- 

 mann n. Raumer, Bot. Zeitung, 1878, p. 209; Btisgen, ibid., 1883, p. 569. 



2 The same may occur in the ampullae of certain hepatics (Jungermannia). Cf. Goebel, 1. c., 

 pp. 185, 209. A few green plants, such as Splachnum, Tetraplodon, only grow upon certain excre- 

 ments. On Cephalotus: Goebel, I.e., pp. 87, 170. Utricularia: Darwin, Insectivorous Plants, 

 T 875> P- 395 ! Goebel, 1. c., p. 173. Biisgen observed that a supply of insects produced a favourable 

 effect (Ber. d. Bot. Ges., 1888, p. lv). 



3 Tischutkin, Bot. Centralbl., 1892, Ed. L, p. 304, and 1893, Bd. LHI, p. 322. See Goebel, 

 I.e., pp. 164, 187, 190. On Drosophyllum, cf. de Wevre, Ann. d. sci. nat., 1895, viii. ser., T. I, 

 p. 51. ^Nepenthes: Vines, Annals of Botany, Vol. xi, 1897, p. 563.] 



4 Goebel, 1. c., p. 193 ; de Wevre, 1. c., p. 39. 



5 Dionaea : v. Gorup Besanez, Ber. d. Chem. Ges., 1876, p. 673. Cf. Vines, Bot. Jahresb., 1876, 

 p. 935. Drosera: Will, Bot. Zeitung, 1875, p. 716. According to Stein (Wunschmann, Uber d. 

 Gattnng Nepenthes, 1872, p. 25), citric acid is present, but according to Darwin (I.e., 1876, p. 78), 

 propionic acid or a mixture of acetic and butyric acids. Fungi can form a variety of acids 

 (Sects. 85, 86). 



6 Volker's analyses (1849) are quoted by Balfour (Gardener's Chronicle, 1875, II, p. 67). 



7 Goebel, I.e., p. 189. On Drosera and Pinguicula, pp. 181, 197; on Drosophyllum, 

 de Wevre, 1. c. 



