18 THE ELEMENTARY STRUCTURE OF PLANTS. 



The chemistry of vegetation may be more advantageously treated 

 of hereafter. As they derive all the materials of their fabric from 

 the earth and air, plants can possess no simple element which 

 these do not supply. They may take in, to some extent, almost 

 every element which is thus supplied. Suffice it for the present to 

 say, however, that, of the about sixty simple substances now recog- 

 nized by chemists, only four are essential to vegetation and are 

 necessary, constituents of the vegetable structure. These are Car- 

 lon, Hydrogen, Oxygen, and Nitrogen. Besides these, a few 

 earthy bodies are regularly found in plants, in small and varying 

 proportions. The most important of them are Sulphur and Phos- 

 phorus, which are thought to take an essential part in the forma- 

 tion of certain vegetable products, Potassium and Sodium, Calcium 

 and Magnesium, Silicon and Aluminum, Iron and Manganese, 

 Chlorine, Iodine, and Bromine. None of these elements, how- 

 ever, are of universal occurrence, or are actual components of any 

 vegetable tissue ; they occur either among the materials which are 

 deposited on the walls of the cells or collected within them. 



11. Their Organic Constitution, Although plants and animals 

 have no peculiar elements, though the materials from which their 

 bodies spring, and to which they return, are common earth and 

 air, yet in them these elements are wrought into something 

 widely different from any form of lifeless mineral matter. Un- 

 der the influence of the principle of life, in connection with 

 which alone any such phenomena are ever manifested, the three 

 or four simple constituents effect peculiar combinations, giving 

 rise to a few organizable elements (27), as they may be termed ; 

 because of them the organized fabric of the vegetable or animal is 

 directly built up. This fabric is in a good degree similar in all 

 living bodies ; the solid parts or tissues in all assuming the form of 

 thin membranes or filaments arranged so as to surround cavities, 

 or form the walls of tubes, in which the fluids are contained. It is 

 called organized structure, and the bodies so composed are called 

 organized bodies, because such fabric consists of parts cooperat- 

 ing with each other as instruments or organs adapted to certain 

 ends, and through which alone the living principle, under whose 

 influence the structure itself was built up, is manifested in the phe- 

 nomena which the plant and animal exhibit. There is in every 

 organic fabric a necessary connection between its conformation 

 and the actions it is destined to perform. This is equally true of 



