THE ESSENTIAL ELEMENTS 421 



always be remembered, however, that the mere change in the composition of the 

 nutrient solution directly affects the growth of the plant. The special requirements 

 of each plant need also to be satisfied and thus many algae may be grown in still 

 water, while others will only grow when the water is in continual motion, and for 

 this and other reasons are not amenable to ordinary culture methods. Obligate 

 marine algae require a saline medium, and hence will not grow in a dilute nutrient 

 solution, but develop only under special cultural conditions l . The nutrition of 

 fungi, and the solid and fluid nutrient media for heterotrophic plants in general, 

 have already been dealt with (Sect. 66). 



The concentration of the culture fluid is always important, for when its osmotic 

 concentration passes a certain limit growth becomes impossible though no 

 poisonous effect is exercised, while when the fluid is too dilute or when a single 

 essential salt is present in insufficient amount the development of the plant is 

 retarded. Hence there is a certain optimal concentration for every nutrient 

 solution, and the presence of any one salt in excessive amount will lower the 

 optimal concentration of the mixture. 



The best nutrient fluid for Phanerogams contains 0-2 to 0-5 per cent, of salts ~, 

 and when the amount is increased to 2-5 per cent., many grow but little or not 

 at all. The same result is produced by adding potassium nitrate or sodium chloride 

 until the osmotic energy of the fluid corresponds to 2 per cent, of the former or 

 1-7 per cent, of the latter, but in certain Phanerogams, and especially in Halophytes, 

 growth ceases only when an osmotic concentration is reached which is equivalent 

 to a 3 per cent, salt-solution 3 . Penicillium and other moulds, as well as certain 

 bacteria, can grow in solutions which correspond in osmotic value to 20 per cent, 

 of potassium nitrate, and certain of the lower algae have similar powers of 

 accommodation, whereas other fresh-water algae are no more resistant than 

 Phanerogams 4 . 



The minimum concentration is not always extremely low, and indeed obligate 

 marine algae can exist only in a strongly saline solution, although various forms 

 may accommodate themselves to brackish and ultimately to fresh water 5 . Many 

 plants probably exist, especially among fungi and bacteria, for which the minimal 

 concentration is comparatively high, as according to Klebs is the case with 

 Rurotium repens ". 



A high minimum is not necessarily correlated with marked accommodatory 



1 Cf. A. Richter, Flora, 1892, p. 9; Klebs, Bot. Zeitung, 1891, p. 789; Die Bedingungen d. 

 Fortpflanzung, &c., 1896; Molisch, Sitzungsb. tl. \Vien. Akad., 1896, Bd. CV, Abth. i, p. 634. 

 Algae as a general rule cannot withstand an acid solution. Oltmanns, Jahrh. f. wiss. Bot., 1892, 

 Bd. XXIII, p. 281, and Flora, 1895, p. 51 ; Noll, Flora, 1892, p. 281. 



- Cf. Nobbe, Versuchsst., 1864, Bd. VI, pp. 40, 3-13, &c. In sand-cultures a higher concentra- 

 tion is possible. Cf. Hellriegel, Jahresb. f. Agr.-Chem., 1861-2, p. 115. 



3 Stange, Bot. Zeitung, 1892, p. 253, and the literature there given. 



* Eschenhngen, Einfluss d. Losungen versch. Cone, auf Schimmelpilze, 1889; Bruhne, in Zopfs 

 Beitrage z. Physiol. u. Moiph., 1894, Heft 4, p. i ; A. Fischer, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, 

 p. 151 bacteria^; Richter, Flora, 1892, p. i 2 ; Stange, I.e., p. 256 algae). 



5 Oltmanns, Monatsb. d. Berl. Akad., 1891, p. 201, and Jahrb. f. wiss. Bot., 1892, Bd. XXIII, 

 p. 405; Flora, 1895, p. 51. 



8 Klebs, Die Bedingungen d. Fortpflanzung. &c., 1896, p. 465. 



