68 NUTRIENT SALTS. 
cules of cellulose that, even after the removal of the latter, the entire structure is 
preserved in outline and in detail. They form, therefore, a regular coat of mail 
which may be looked upon as a means of protection against certain injurious ex- 
ternal influences. 
For a large number of plants living in the sea, sodium, iodine, and bromine also 
are of especial importance as food-stufts. How far fluorine, manganese, lithium, 
and various other metals, which have been detected in the ash of some plants, are 
of use is not determined, for our knowledge is particularly incomplete with respect 
to the various uses subserved in nutrition and growth by the different mineral 
food-stufts. It is worthy of note that alumina, which is so widely distributed and 
easily accessible to plants, is only very rarely absorbed. The ash of Lycopodiwm 
is the only kind in which this substance has been identified with certainty in any 
considerable quantities. 
Lastly, amongst the sources of elements contained in the food-salts, we must 
consider the solid crust of the earth. But it is only in the case of comparatively 
few vegetable organisms that this earth-crust forms the immediate foster-soil. 
The majority derive the salts that nourish them from the products of the weather- 
ing of rocks, from refuse and the decaying remains of dead animals and plants, 
which, in decomposing, give back their mineral substances to the ground, from 
underground waters that filter through fissures in rocks and through the interstices 
of sandy or clayey soils soaking with lye, the adjacent parts of the earth’s crust, 
and, lastly, from the water of springs, streams, ponds, and lakes, which have come 
to the surface holding salts in solution, as also from sea-water with its rich supply 
of salts. 
The very salts that are needed by most plants are amongst the most widely 
distributed on the earth’s surface. The sulphates of calcium and of magnesium, 
for example, and salts of iron, potassium, &e., are found almost everywhere in the 
earth, and in water, whether subterranean or superficial. At the same time it is 
very striking that these mineral food-salts are not introduced into plants by any 
means in proportion to the quantity in which they are contained in the soil, but 
that, on the contrary, plants possess the power of selecting from the abundance of 
provisions at their disposal only those that are good for them and in such quantity 
as is serviceable. This selective capacity of plants is manifested ın many ways, and 
we will now briefly consider some of the most important of them. 
In the first place we have the fact that plants reared close together in the same 
soil or medium may yet exhibit an altogether different composition of ash. This 
is particularly striking in water and bog-plants, which, though rooted in close 
proximity and immersed in the same water, show very considerable differences in 
respect of mineral food absorbed. The result, for instance, of testing specimens of 
the Water-soldier (Stratiotes aloides), the White Water-lily (Nymphea alba), 
a species of Stone-wort (Chara fetida), and the Reed (Phragmites communis), all 
growing close together in a swamp, was as follows as regarded the potash, soda, 
lime, and silieie acid, held by them respectively :— 
