422 THE FOOD OF PLANTS 



powers, for it is just those plants which can withstand the greatest concentration 

 that can also accommodate themselves to the most dilute solutions. The question 

 is not entirely a matter of turgidity, although an organism can accommodate itself to 

 a concentrated solution only when it is able to increase the osmotic strength of the 

 cell-sap to a corresponding extent. It has already been shown (Sect. 24) that this is 

 partly attained by the penetration of the surrounding salts, and partly by a regulatory 

 production of osmotically active substances. A rapid power of accommodation is 

 possessed by many fungi and bacteria, as well as by those algae which are exposed 

 to rapid changes from sea-water to fresh-water, but nevertheless a sudden difference 

 in the concentration appears always to temporarily diminish the rate of growth '. 

 Fungi may withstand a sudden diminution of concentration in the surrounding 

 medium equivalent to a 5 per cent, solution of sodium chloride, but when the 

 change is still more pronounced, mechanical ruptures may be produced owing to 

 the enormous internal osmotic pressure coming freely into play. 



In a concentrated nutrient solution many algae and fungi show a tendency to 

 fragmentation, whereas the stem of Phanerogams usually becomes more or less 

 condensed, and develops thicker and more fleshy leaves a . The influence of the 

 solution is not merely due to its osmotic concentration, but the salt present appears 

 to exercise a specific effect, for a solution of sodium chloride produces a more 

 marked result than an isosmotic solution of potassium nitrate s . 



SECTION 74. The Functions of the Essential Elements. 



The ash constituents are of importance only when they take part in 

 metabolism, and hence the functions which any essential element, such as 

 potassium or phosphorus, has to perform, are probably very varied in 

 character, and cannot be determined until we know the entire series of 

 processes in which the given element may take part from the time when 

 it is first absorbed until it becomes of no further use to the plant. This is 

 the ideal that physiology strives after, but our knowledge of the internal 

 vital processes is at present extremely slight, and hence when we regard the 

 living organism from a superficial stand-point we attach great importance to 

 the carbon-compounds which preponderate in it, while the function of the 

 ash constituents attracts less attention and may even be purposely neglected. 

 So little is known of the parts played by the different mineral constituents 

 that it is impossible to give a connected account of their importance in 

 metabolism, and hence it must suffice to give the fragmentary information 

 which has been gained concerning the importance of individual ash con- 

 stituents and the changes they may undergo. 



The fact that a particular element is an essential one gives no indication 



1 True, Annals of Botany, 1895, Vol. IX, p. 369; also see Eschenhagen, Stange, and Ricliter. 



a For examples, see Eschenhagen, Stance, Kichter, Klebs; also Schimper, Indoraalayische 

 Strandflora, 1891, p. 26; Dassonville, Rev. gen., 1896, pp. 284, 324. On the influence upon 

 starch-formation and CO 2 -assimilation, cf. Sects. 55 and 58; on transpiration, Sect. 39. 



3 Stange, 1. c., p 366. 



