THE ASH OF PLANTS. 197 



single source of sulphur to plants. The sulphurized oils 

 of the onion, mustard, hprseradish, turnip, etc., likewise 

 require sulphates for their organization. 



Phosphates. The phosphorizfcd oils (protagon) require 

 to their elaboration that phosphates or some source of 

 phosphorus be at the disposal of the plant. The physio- 

 logical function of the phosphates, so abundant in the ce- 

 reals, admits of partial explanation. The soluble albumi- 

 noids which are formed in the foliage must pass thence 

 through the cells and ducts of the stem into growing parts 

 of the plant, and into the seed, where they accumulate in 

 large quantity. But the albuminoids penetrate membranes 

 with great difficulty and slowness when in the pure state. 

 According to Schumacher, (Physik der Pflanze, p. 128,) 

 the phosphate of potash considerably increases the diffu- 

 sive rate of albumin, and thus facilitates its translocation 

 in the plant. 



Alkalies and alkali-earths. The organic acids, viz. : 

 oxalic, malic, tartaric, citric, etc., require alkalies and al- 

 kali-earths to form the salts which exist in plants, e. g. bi- 

 tartrate of potash in the grape, oxalate of lime in beet- 

 leaves, malate of lime in tobacco ; and without these bases 

 it is, perhaps, in most cases impossible for the acids to be 

 formed, though in the orange and lemon, citric acid exists 

 in the uncombined or free state, and in various plants, as 

 Sempervivum arboreum, and Cacalla ficoides, acids are 

 formed during the night which disappear in the day. The 

 leaves of these plants are sour in the morning, tasteless at 

 noon, and bitter at night. (Heyne & Link).) 



Silica. The function of silica might appear to be, in case 

 of the grasses, sedges, and equisetums, to give rigidity to 

 the slender stems of these plants, and enable them to sustain 

 the often heavy weight of the fruit. Two circumstances, 

 however, embarrass the unqualified acceptance of this no- 

 tion. The first is, that the proportion of silica is not great- 



