NUTRIENT SALTS. 67 
It is obvious that food-salts can only pass through cell-membranes and reach 
the interior of a plant in a state of solution. On this account the soluble sul- 
phates, phosphates, nitrates and chlorides of calcium, magnesium, potassium and 
iron, may pre-eminently be called food-salts. Whether an essential element is 
absorbed by a plant in the form of one of these compounds or another appears 
to be unimportant; phosphorus, for example, may be proffered by the soil in the 
form either of potassium phosphate or of sodium phosphate, with like results. 
As regards the importance of sulphur to plants, it is at any rate established that 
it is necessary for the production of proteid substances. Phosphorus appears to be 
indispensable in the transformation of certain compounds of nitrogen. Potassium 
is supposed to play a part in the formation of starch. Calcium is introduced into 
plants in combination with sulphuric acid as calcium sulphate. This salt is decom- 
posed, the lime combining with oxalic acid to form insoluble calcium oxalate, and 
the sulphur going to form the sulphuric acid which is used in the construction of 
albuminous substances or proteids. Lime is therefore important, inasmuch as 
it is a medium of transport for sulphur. Iron certainly participates in the forma- 
tion of chlorophyll, even if it does not enter into its composition, as was formerly 
supposed. For, it has been proved, by means of artificial cultures, that plants reared 
in solutions free from iron were white instead of green, and died at last; whereas, 
after the addition of a small quantity of a soluble iron salt, such plants became green 
in a very short time, and were able to continue their development. The utility of 
most of these elements does not therefore appear to consist necessarily in their 
entering into the composition of organic compounds, but in the promotion and 
regulation of the constructive and destructive chemical processes. 
Silicic acid, which occurs so plentifully in the ash of many plants as to con- 
stitute often more than 50 per cent, has a different function. If the minute 
unicellular water-plants known as Diatoms are incinerated, or if stems of Equisetum, 
Juniper-needles, or leaves of grasses, &e., are subjected to a red heat, white skeletons 
remain behind which consist almost entirely of silicic acid, and exhibit not only 
the forms of the cells, but even the finest sculpturing of the cell-walls. In par- 
ticular, the stiff hairs on the leaves of grasses are preserved, and better still the 
cell-membranes of diatoms. The latter present very beautiful forms with their 
outlines quite distinct, and many structural properties of the cell-membranes, 
especially their moulding, striation, and the dots and other exerescences are to be 
seen much more clearly after than before ignition, when the transparency was less 
owing to the protoplast occupying the interior of each cell. In order to describe 
exactly the very varied form of Diatomacez, specimens are carefully and thor- 
oughly ignited, and the descriptions and illustrations of these microscopic plants 
are for the most part made from siliceous skeletons prepared in this way. These 
skeletons show clearly that silicie acid occurs only in the cell-membrane, and plays 
no part as constituent of any chemical compound in the protoplasm; nor does it 
appear to be instrumental in the formation of any such compound. The molecules 
of silicic acid are so closely packed and so evenly distributed amongst the mole- 
