GS NCTiüExr sAi/rs. 



cules oi' cellulose that, even al'ter the i'enioval of the latter, tlie entire structure is 

 preserved in outline and in detail. They form, therefore, a regular coat of mail 

 M'hich may be looked upon as a means of protection against cei-tain injurious ex- 

 ternal influences. 



For a large number of plants living in the sea, sodium, iodine, and bromine also 

 are of esjjecial importance as food-stuflfe. 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 Lycopodiimn 

 is the only kind in which this substance has been identified with certainty in anj' 

 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 comparativelj' 

 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 decajäng remains of dead animals and plants, 

 which, in decomposing, give back their mineral substances to the gi-ound, 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 widelj- 

 distributed on the earth's surface. The sulphates of calcium and of magnesium, 

 for example, and salts of iron, potassium, &c., are found almost eveiywhere 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 anj- 

 means in proportion to the quantity in which thej' 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 (juantitj- 

 as is serviceable. This selective capacity of plants is manifested ni 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 diftereut composition of ash. This 

 is particularly striking in water and bog-plants, which, though rooted in close 

 pi-oximity and immersed in the same water, show verj- considerable differences in 

 respect of mineral food absoi-bed. The i-esult, for instance, of testing specimens of 

 the Water-soldier (Straiiotes aloides), the White Water-lily (Fymphcea alba), 

 a species of Stone-wort (Chara fcrfiria), 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 respectivel j' : — 



