668 



Anyone who has cultivated fungi in nutrient solutions knows, however, 

 how favorably a supply of sugar acts on the development of many parasitic 

 fungi. 



Cloudy, cool days, therefore, not only weaken the assimilation in the 

 green parts of the plants but, at the same time, by reducing the respiratory 

 processes, bring about an accumulation of sugar in the leaf cells and, there- 

 fore, make possible the production of a more favorable substratum for 

 parasites. 



The acid content of the various plant parts is also very different in 

 the dark from that found when the organ is favorably illuminated. 



The observation, that many plants (Crassulaceae) taste sour at night' 

 but not noticeably so during the day- is very odd. In etiolated plants, Wies- 

 ner could recognize an abundance of organic acids^ in the leaves of many 

 monocotyledons, and later De Vries observed* that the stems of etiolated 

 dicotyledons are strongly acid. When illuminated, the rich sugar content 

 disappears. This has been, at least, especially proved for the Crassulaceae, 

 in which, in the night, De Vries could determine a rich acid formation only 

 when the plants had been abundantly lighted during the day, but, if the 

 supply of light was limited to a few hours, the acid content in the night was 

 correspondingly less. 



An increase of warmth increases also the decomposition of the acids in 

 the dark. Cooler nights lead to the storage of acid. 



De Vries has proved this directly by experiments^ It is evident, how- 

 ever, from the fact that the loss of acid becomes less with each successive 

 day of shading, that the disappearance of the acids is connected with the 

 supply of material for the formation of acid which has been worked over 

 in the light. 



Plants, therefore, constantly produce acids and the more energetically 

 the stronger growing the organs are. With light, the acids are oxidized as 

 fast as they are produced ; in the dark, they are stored up. On this account, 

 etiolated plants are relatively rich in acids. The suppression of the inflor- 

 escences increases the content of free acids in the leaf. The acid content 

 in the roots is also subjected to great fluctuations and, according to Chara- 

 bot", in plants cultivated in the shade it is, in fact, larger than in the leaves. 

 In general, this acid content is greater in etiolated plants. 



This accumulation of acids in and of itself can offer those fungi, which 

 decompose acids, the possibility of colonization and luxuriant development ; 



1 Heyne und Link in Jahrbuch der Gewachskunde von Sprengel, Schrader und 

 Link, 1819, p. '70-73. 



2 Ad. Mayer, Uber Sauerstoffausscheidung usw. Verhandl. d. Heidelberger 

 naturf. Gesellsch. 4-8, 1875. Landwirtsch. Versuchsstat. 1875, Vol. XVIII, p. 410, 

 Vol. XXI, p. 277. 



3 Wiesner, Sitzungsber. d. K. K. Akad. d. Wissensch. I, April, 1874, Vol. 69; 

 cit. Bot. Zeit. 1874. p. 116. 



4 De Vries, Uber die Bedeutung der Pflanzensauren fur den Turgor der Zellen. 

 Bot. Zeit. 1878, p. 852. tjber die periodische Saurcbildung der Fettpflanzen. Bot. 

 Zeit. 1884, Nos. 22 and 23. 



5 Bot. Zeit. 1884, p. 340. 



6 Charabot, E., et Herbert, A., Rechercbes sur I'acidite v6getale. Compt. rend. 

 1904, CXXXVIII, p. 1714. 



