374 THE FOOD OF PLANTS 



exert a pressure of from eight to fifteen atmospheres, and when no lateral 

 divergence is possible, may penetrate leaves, stems, and other living organs 

 merely by means of this mechanical pressure l . The penetration of the 

 haustoria of Cttscnta is, however, apparently aided by the action of cellulose- 

 dissolving enzymes, and this is perhaps the case in all phanerogamic 

 parasites. 



In Ciiscuta contact-stimuli induce the formation of haustoria, and from 

 these the penetrating absorbent organs develop. Similarly special attaching 

 organs are developed in many fungi (Botrytis cinerea), while whenever the 

 growing apex exerts marked pressure in boring through a membrane, 

 the older portion of the hypha must adhere with considerable tenacity. 



A fungus which has at the outset been unable to penetrate a leaf may acquire 

 this power when the injury of a few internal cells causes attractive juices to exude 

 from them through the leaf. The thickness and character of the separating 

 membranes are of course as important as are the special peculiarities of the 

 attacking fungus, while even when penetration is possible the hyphae may not find 

 suitable conditions for their development and may perish. In every case several 

 factors are at work, as is shown by the restriction of certain parasites to particular 

 hosts. No discussion of these interesting and complicated problems is however 

 possible, for a precise determination of the different, co-operating factors has not 

 as yet been made in a single case 2 . All infectious diseases present similar 

 problems for solution. 



The excretion of solvent enzymes. Especially in heterotrophic plants 

 secretory products are often used to dissolve solid materials, or to render 

 soluble substances capable of more rapid diosmosis and absorption. In 

 addition to acids and alkalies, enzymes or ferments are frequently employed, 

 the latter being especially useful, since a very small amount suffices to 

 transform a very large quantity of fermentable material. For the present 

 we are concerned only with the importance of such secreta as a means to 

 obtain nutriment, but ferments often take part in metabolism, and hence 

 further mention will be made of them (Sect. 91). 



In fungi and bacteria the extracellular action of enzymes is utilized 

 in a variety of ways, as, for example, in the penetration of cell-walls by 

 means of cellulose-dissolving enzymes, while fungi which grow upon insects 

 (Emptisa, Entomopktkora, &c.) excrete an enzyme which dissolves chitin 3 . 



1 Pfeffer, Dmck u. Arbcitsleistung, 1893, p. 372 ; Peirce, Bot. Zeitung, 1894, p. 169. 



2 Cf. de Bary, Pike, 1884, p. 420, and Bot. Zeitung, 1886, p. 441 ; Miyoshi, 1. c., 1895 ; Brefeld, 

 Unters. a. d. Gesammtgebiete d. Mykologie, 1895, XI, p. 90 ; Erikson, Zeitschr. f. Pflanzenkrank- 

 heiten, 1895, v, p. 80. 



3 Miyoshi, jahrb. f. wiss. Bot., 1895, Bd. XXIX, p. 277. Literature by de Bary, Pilze, 1884, 

 p. 399; Zopf, ibid., 1890, p. 180. 



