SECT. II PHYSIOLOGY 181 



are the non-metals CI, S, P, Si, and the metals K, Na, Ca, Mg, 

 and Fe. 



Besides these tlie following may be met with in the ash of plauts : Iodine, 

 Bromine, Fluorine, Selenium, Tellurium, Arsenic (which may be introduced into 

 cultivated soils with superphosjihates). Antimony, Tin, Titanium, Boron, Lithium, 

 Rubidium, Strontium, Barium, Zinc, Copper, Silver, Mercury, Lead, Aluminium, 

 Thallium, Chromium, Manganese, Cobalt, and Nickel. 



Organic Substance. — Chemical analysis is not needed to show 

 that the plant contains carbon in a combined form. Every burning 

 log or match shows by its charring that it contains carbon. The 

 examination of a piece of charcoal in which the finest structure of the 

 wood is retained, shows further how uniformly the carbon is distributed 

 in the plant, and how largely the substance of the plant consists of this 

 element. Accurate weighing has shown that carbon constitutes about 

 one-half of the dry weight of the plant. The enormous masses of coal 

 in the earth's crust are the carbonised remains of plants that lived in 

 earlier geological periods ; lignite and peat and also coal, when pre- 

 pared by special methods, exhibit their vegetable origin by their 

 macroscopic and microscopic structure. 



On combustion of the dry plant the organic substance is changed, 

 and passes off in the form of carbon dioxide and water, ammonia or 

 free nitrogen. It contained the elements H, 0, N, and C chemically 

 combined ; some of the elements mentioned as occurring in the ash 

 may also occur in organic compounds. 



Source of the Materials. — There are thus oplYJ liirteen eleme nts, 

 found in considerable quantity in the plant. When the plant is 

 growing their amount is continually increasing in the plant, and 

 they must therefore be continually absorbed from without. 



The mode of life of a plant affords clear indications as to the 

 source of the materials from which it is built up. Plants are known 

 which live wholly in the soil ; others float freely in water ; others, 

 while attached to a substratum, are eventually dependent on the air ; 

 while, lastly, others live upon the bodies of other plants or of animals 

 (parasites). 



Thus the substances which are found within the plant must have 

 been derived from the soil, the water, the air, or from other organisms. 

 Rarely, however, does a plant obtain all that it requires from one 

 only of these media. The ordinary terrestrial plant sends its root 

 down into the soil, and expands its leafy shoots in the air. Many 

 aquatic plants have, in addition to the submerged organs, others which 

 reach the atmosphere. Parasites also are able to absorb substances 

 from the atmos})here. 



As a rule, only gases and liquids can enter the plant ; solid 

 substances have to be brought into solution before they can pass 

 through the firm cell-walls. When, however, cell-walls are absent, as 



