192 HOW CROPS GROW. 



Another class of plants the sea- weeds (algce) de- 

 rive their nutriment exclusively from the sea-water in 

 which they are immersed. Though the quantity of po- 

 tassium in sea-water is but ^ that of the sodium, it is 

 yet a fact, as shown by the analyses of Forchhammer 

 (Jour, fur Prakt. Chem., 36, p. 391) and Anderson 

 (Trans. High, and Ag. Soc., 1855-7, p. 349) that the 

 ash of sea-weeds is, in general, as rich, or even richer, in 

 potassium than in sodium. In 14 analyses, by Forch- 

 hammer, the average amount of sodium in the dry weed 

 was 3.1 per cent; that of potassium 2.5 per cent. In 

 Anderson's results the percentage of potassium is inva- 

 riably higher than that of sodium.* 



Analogy with land-plants would lead to the inference 

 that the sodium of the sea-weeds is in a great degree ac- 

 cidental. In fact, Fucus vesiculosis and Zygogonium sal- 

 inum have been observed to flourish in fresh water. 

 (Vs. St., XIII, p. 295.) 



Iron is Essential to Plants. It is abundantly 

 proved that a minute quantity of ferric oxide, Fe 2 8 , is 

 essential to growth, though the agricultural plant may 

 be perfect if provided with so little as to be discoverable 

 in its ash only by sensitive tests. According to Salm- 

 Horstmar, ferrous oxide, FeO, is indispensable to the 

 colza plant. (Versuche, etc., p. 35.) Knop asserts that 

 maize, which refuses to grow in entire absence of iron, 

 flourishes when ferric phosphate, which is exceedingly 

 insoluble, is simply suspended in the solution that bathes 

 its roots for the first four weeks only of the growth of 

 the plant. ( Vs. St., V, p. 101.) 



We find that the quantity of ferric oxide given in the 

 analyses of the ashes of agricultural plants is small, being 

 usually less than one per cent. 



Here, too, considerable variations are observed. In 



* Doubtless iluc to the fact that ilie material used by Anderson was 

 freed by washing from adhering common salt. 



