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, &c, 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 excrescences 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 Diatomacere, 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 silicic 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- 



