1 90 Elementary Biology. 



contain a much larger percentage of water than terrestrial or 

 aerial plants, though there are numerous exceptions to this 

 rule. In spring, when active growth is commencing, the 

 proportion of water rapidly increases. The younger parts of 

 the plant also contain more water than older parts. 



The dried or desiccated plant may further be subjected 

 to calcination, or red heat, when the greater proportion of 

 the solid residue is removed in the form of simple com- 

 pounds, such as carbonic acid, water of combination, and 

 ammonia. The elements found to be present by an exami- 

 nation of the products of calcination are carbon, hydrogen, 

 oxygen, and nitrogen ; whilst the ash left over contains, as 

 a general rule, sulphur, phosphorus, chlorine, silicon, potas- 

 sium, calcium, magnesium, sodium, and iron, all in the form 

 of simple salts, or even as bases or oxides. Many other 

 substances are found in particular plants, those, that is 

 to say, growing in certain districts where such substances 

 occur in the soil, or where the plant has peculiar facilities 

 for the absorption of such substances. These accidental 

 constituents are iodine and bromine, which are found in 

 many marine plants, lithium, zinc, copper, aluminium, man- 

 ganese, cobalt, nickel, strontium, and barium. 



It will be convenient to classify these elements in the 

 order of their relative importance, thus : 



I. Essential elements present in the plant organism in 

 large amount. (C, H, O, and N.) 



II. Essential elements present in small amount. (S, P, 

 K, Ca, Mg, Fe.) 



III. Non-essential elements present in variable amount, 

 and not in all plants. (I, Br, Zu, Cu, Al, Mn, Co, Ni, Sr, 

 Li, Ba, Si, Na, Cl.) 



I. Essential elements in the plant organism, present 

 in large amount. 



CARBON. 



Origin. The carbon is obtained from the carbonic acid 



