278 ALKALIES. [BOOK 11. 



It is, however, by no means certain that the alkalies may 

 be substituted for each other in maintaining vegetation. 

 The following case, mentioned by Professor Moretti, seems 

 to indicate the contrary : 



Let two pots of the same earth, under circumstances the 

 same in all respects, be planted, the one with common Pelli- 

 tory (Parietaria qfficinalis), the other with common Saltwort 

 (Salsola Kali) . Let them be both watered equally with two 

 different solutions, the one of salt (muriate of soda), the other 

 of saltpetre (nitrate of potash) . Although each will have its 

 roots equally exposed to the salt and saltpetre, yet the Pelli- 

 tory will be found to contain saltpetre without salt, and the 

 Saltwort, salt without saltpetre. It therefore seems clear 

 that the roots of Saltwort refuse to imbibe saltpetre, and 

 those of Pellitory to feed on salt. It is true that the above 

 fact may be apparently explained by supposing that Saltwort 

 rejects potash, and Pellitory soda, after they have imbibed 

 them ; but if this really happens, we nevertheless seem forced 

 to conclude that soda is the alkali necessary to Saltwort, and 

 potash that essential to Pellitory. 



About the general importance of alkalies to plants there 

 seems, however, to be no room for doubt, and the discovery of 

 Liebig explains many things that were inexplicable before. 

 When we say that a plant becomes tired of a soil, and find 

 that manuring fails to invigorate it, the destruction of alka- 

 lies in the soil, and the want of a sufficient supply of those 

 bases in the manure, seem to offer a solution of the enigma. 

 And in like manner the gradual decay of trees in public 

 squares and promenades, where the soil is incessantly robbed 

 of alkaline matter, for the sake of neatness, may probably be 

 ascribed to the same cause. So also the injurious action of 

 weeds is explained by their robbing the soil of that particular 

 kind of food which is necessary to the crops among which 

 they grow. Each will partake of the component parts of the 

 soil, and in proportion to the vigour of their growth, that of 

 the crop must decrease ; for what one receives the others are 

 deprived of. "Plants will, on the contrary, thrive beside 

 each other, either when the substances necessary for their 

 growth, which they extract from the soil, are of different 



