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-stuffs. 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-stuffs. 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 Lycopodium 

 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, &c., 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 in 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 (Nymphcea alba), 

 a species of Stone-wort (Chara fatida), and the Reed (Phragmites communis), all 

 growing close together in a swamp, was as follows as regarded the potash, soda, 

 lime, and silicic acid, held by them respectively: 



