EXTERNAL INFLUENCES 513 



many other of the lower organisms are able to conquer and kill their adversaries 

 by means of certain poisonous excretory products, while by means of cellulose- 

 enzymes parasitic fungi are able to penetrate a host-plant. In reciprocal symbiosis, 

 however, the exchanges are of mutual advantage. 



Even the normal excretory bye-products may injuriously affect other organ- 

 isms : thus 2 to 5 per cent, of alcohol acts injuriously or fatally upon many 

 organisms, although certain alcohol-producing species of Saccharomyces can with- 

 stand 14 per cent. (Sect. 103). Further, most bacteria and many other organisms 

 are injured by even a slightly acid medium, and hence are suppressed and killed 

 by fungi which produce large quantities of free acid without being themselves 

 affected l . Thus if yeast and bacteria are sown in an acid solution of sugar the 

 yeast attains the upper hand, whereas in an alkaline or neutral solution the bacteria 

 rapidly develop and multiply, and the yeast is suppressed 2 . 



The final result in all such cases depends upon the specific properties and 

 reactive powers of the different organisms, as well upon the external conditions, 

 that is upon several variable factors. Thus the amount of organic acid produced 

 is liable to modification, and the power of secreting poisons may be entirely sup- 

 pressed, so that both the means of attack and defence are subject to alteration. 

 A parasite may penetrate a host only under special conditions, and it is possible 

 to induce the penetration of many fungi which are not parasitic. The acidity of 

 the cell-sap probably suffices to prevent the multiplication of injected bacteria and 

 gradually to kill them 3 ; certain metabolic products, such as those found in many 

 varieties of latex, may also exercise a directly poisonous effect. 



All plants possess more or less marked powers of accommodation, and thus 

 a plant may gradually accustom itself to more highly concentrated media or to 

 doses of poison which were previously fatal. Hence, whether an antagonistic 

 symbiont excretes an injurious product rapidly or slowly, is a point of some 

 importance, for when the latter is the case, the plant attacked has a longer time to 

 accustom itself to the poison, or to prevent an injurious accumulation by destroying 

 or neutralizing it. Thus in certain cases excreted oxalic acid may be decomposed 

 or combined with bases and rendered innocuous. Similarly, although a slight 

 amount of methyl-violet suffices to kill protoplasm, considerable amounts may 

 slowly pass through the protoplast and acccumulate in the cell-sap in the form of 

 a non-diosmosing and hence innocuous compound, when it is presented in extreme 

 dilution. 



A plant may also make a counter-attack upon its enemy, as, for example, by 

 means of such secretions as weaken the energy and poisonous character of the 

 attacking organism. Substances are frequently produced by reciprocal and 

 antagonistic interactions, which the isolated individual plants are unable to form, 



1 Cf. Sect. 85. Wehmer, Beitr. z. Kenntniss einheim. Pilze, 1893, p. 69; 1895, p. 143; Rein- 

 hardt, Jahrb. f. wiss. Bot., 1892, Bd. xxm, p. 515. 



" Niigeli, Die niederen Pilze, 1877, p. 31; Pasteur, Ann. d. chim. et d. phys., 1858, iii. ser., 

 T. LIT, p. 415. The antagonistic action is due to the products, and not to any molecular action 

 connected with fermentation. 



a Cf. Zinsser, Jahrb. f. wiss. Bot., 1897, Bd. xxx, p. 425. 



I'FEFFER L 1 



