ASHES OF PLANTS. 'NOURISHMENT OF AERIAL PLANTS. 121 



there is a considerable amount of phosphate of lime, by which 

 their power of nourishing Animals is greatly increased, since this 

 substance constitutes the earth of bones. 



1 70. Of these different mineral ingredients, each Plant seems 

 to have some one or more, that are as essential to its growth, as 

 is any other article of its food; but the quantity required is 

 sometimes extremely minute, so as to be scarcely detectible, 

 only a very small quantity of ash remaining, after the tissue has 

 been burned. In other instances, again, the mineral matter is 

 so abundant, as to present itself in the form of large crystals, 

 which are deposited between the cells of the tissue. But that 

 which seems its proper office, is to form part of the membranous 

 walls of every cell and tube of the whole structure, however 

 delicate these may be. If a thin portion of almost any plant, be 

 burned in such a manner, that free combustion of all its gaseous 

 elements may take place, without disturbing the place of those 

 which remain, a beautiful skeleton, consisting of extremely 

 minute particles of mineral matter, will be seen, in which the 

 form of all the cells, vessels, &c. may be distinctly traced. These 

 particles would seem to be dispersed throughout the minutest 

 parts of the vegetable tissue ; and they probably serve the pur- 

 pose of conferring additional strength upon the delicate frame- 

 work of which it consists. Even in the finer ashes left by the 

 combustion of common coal, a person to whom the forms of the 

 elementary tissues of plants are familiar, will often succeed in 

 detecting with the microscope, fragments of such skeletons ; 

 which thus add to the evidence otherwise sufficiently strong, 

 of the Vegetable nature of that substance. 



171. Now that we arc acquainted, therefore, with the ele- 

 ments of which the Vegetable structure is composed, and have 

 some knowledge of the sources whence these are derived, we are 

 prepared to inquire more minutely, into the manner in which 

 they are severally received into the organism, and made parts of 

 its structure. This is an inquiry of the highest consequence in 

 Agriculture, an art which, as it has been justly observed, is 

 superior in importance to every other, since on it Man entirely 

 depends for his subsistence, and in great part also for the wealth 



