18 THE ELEMKNTARY STRUCTURE OF PLANTS. 



As they derive all the materials of their fabric from the earth and 

 air, plants can possess no simjile 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, that, of the about 

 sixty simple substances now recognized by chemists, only four are 

 essential to vegetation and are necessary constituents of the vege- 

 table structure. These are Carhon, Hydrogen, Oxygen, and Nitro- 

 gen. Besides these, a few earthy bodies are regularly found in 

 plants, in small and varying proportions. The most important of 

 them are Sulphur and Phosphorus, which are thought to take an 

 essential part in the formation of certain vegetable products. Potas- 

 sium and Sodium, Calcium and Magnesium, Silicon and Aluminum, 

 Iron and Manganese, Chlorine, Iodine, and Bromine. None of these 

 elements, however, are of universal occurrence, nor are they actual 

 components of any vegetable tissue. 



11. Their Organic Constitution., Although plants and animals have 



no pecuhar elements, though the materials from which their bodies 

 spring, and to Avhich they return, are common earth and air, yet in 

 them these elements are wrought into something Avidely different 

 from any form of lifeless mineral matter. Under the influence of 

 the principle of life, in connection with which alone such phenomena 

 are manifested, the three or four siinple constituents effect peculiar 

 combinations, giving rise to a few organizable elements, as they 

 may be termed ; because of them the organized fabric of the vege- 

 table or animal is directly built up. This fabric is in a good degree 

 similar in all hving bodies ; the solid parts, or tissues, in all assuming 

 the form of membranes, 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 pai-ts co-operating 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 operations Avhich the plant 

 and animal carry on. 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 the minute structure, or tissues, as 

 revealed by the microscope, and of the larger organs which the tissues 

 form in all plants and animals of the higher grades, such as a leaf, 

 a petal, or a tendril, a hand, an eye, or a muscle. The term organ- 

 ization formerly referred to the possession of organs in this larger 



