556 PHYSIOLOGY. 



Analyses of plants by chemical means, and in some instances by 

 the spectrum, have demonstrated the existence of the following che- 

 mical elements in plants : Oxygen (0), Hydrogen (H), Carbon 

 (0), Nitrogen (N), Chlorine (Cl),' Bromine (Br), Iodine (I), Fluorine 

 (F), Sulphur (S), Phosphorus (P), Silicon (Si), Potassium (K), 

 Sodium (Na), Calcium (Ca), Magnesium (Mg), Aluminium (Al), 

 Manganiuin (Mn), Iron (Fe), Zinc (Zn), Copper (Cu), Lead (Pb), 

 Titanium (Ti), Arsenic (As), Lithium (Li), Rubidium (Ifcb), Csesium 

 (Cso), Strontium (Sr), and Barium (Ba). 



All of these, however, do not exist in every vegetable substance ; the 

 first four are universally present, while a perfectly healthy condition can- 

 not be assured unless sulphur, potassium, calcium, magnesium, iron, and 

 phosphorus are also present at some time or other. Some of these substances 

 are dissipated by burning, others remain after burning and constitute the 

 ash ; such are the earthy alkalies and metallic substances. The propor- 

 tion of ash ingredients is about 3-6 per cent, of dry matter, i. e. matters 

 dried till they cease to lose weight at a temperature of 100 C. 



Proportionate Quantities of the Constituents, These elements 

 are not taken up by plants in a simple form ; and none of them 

 exist as such in. vegetable substances. The compounds of the 

 different elements differ much in the proportion in which they 

 exist. Water (HO or H 2 O) may form 90 to 95 per cent. Of the 

 dry substance, compounds of carbon, hydrogen, and oxygen (C, H, 

 0) may form 66 per cent. ; the carbon furnishing about 50 per 

 cent., the oxygen about 33 per cent., the hydrogen about 5 per 

 cent., the nitrogen |-4 per cent. ; the alkalies, earths, and metallic 

 oxides commonly form 1 to 4 per cent., in rare cases as much as 

 20 per cent. 



The great mass of all plants is composed of the first four elements in 

 the list the solid parts of compounds of carbon, hydrogen, and oxygen ; 

 the protoplasmic cell-contents of compounds of these three elements, 

 with the addition of nitrogen. Sulphur and phosphorus appear to be 

 necessary constituents also in the protoplasmic compounds ; the alkalies 

 and earths are, in most cases, requisite in the processes of elaboration, 

 but may, in many cases, be substituted for one another, and perhaps in 

 certain cases may be replaced by ammonia. Potash is an indispensable 

 element in plant-growth its presence being essential in the formation of 

 starch from chlorophyll. Chlorine is necessary in many plants : iodine 

 and bromine are also met with, particularly in marine plants ; but it is 

 not clear whether their presence is necessary, or merely an inevitable 

 result of the absorption of sea- water. Iron and manganese are met with 

 very commonly, iron being essential to the formation of chlorophyll, and 

 therefore of the utmost consequence to plant-growth ; copper and zinc 

 more rarely ; silica abounds in certain Orders (Grasses, Equisetaceae), and 

 is met with in many plants in smaller proportions. The most necessary 

 ingredients for the clue nutrition of the plant are, in various proportions 



