THE PHYSIOLOGY OF NUTRITION 135 



Silicon. This element is taken in by the roots of plants in the form 

 of soluble silicates. It is very largely deposited in cell-walls, and 

 probably protects the plant from the attacks of fungi which are unable 

 to penetrate through external walls in which silicon is present. Wheat 

 and all the cereal grains have a very large quantity of silicon in their ash ; 

 the ash of wheat-straw contains as much as seventy per cent, of silicon. 



Sodium. Sodium is one of the most widely distributed of all the 

 elements, and it is no wonder that it is contained in the ash of all plants. 



Chlorine. The ash of all plants contains a little chlorine, but it is 

 not essential for their nutrition. Buck-wheat, Barley and Oats seem 

 to grow better if they are supplied with chlorine ; Maize will grow in 

 solutions without it. Chlorine and sodium, in the form of common 

 salt, seem to keep plants healthy. 



SUMMARY. 



Physiology. The division of botany which deals with what a plant 

 can do is termed physiology. All the higher plants show division of 

 labour, i.e., each part of the plant performs a special kind of work. 



Nutrition.- The processes which enable a plant to obtain and 

 change its food, thus enabling the plant to form new tissue, is spoken 

 of as nutrition. 



The Essential Elements of Plant Food are 



I. Carbon 2. Hydrogen 3. Oxygen 4, Nitrogen 



5. Sulphur 6. Phosphorus 7. Potassium 8. Calcium 



9. Magnesium 10. Iron 



Water Culture. When a plant is grown in a solution the ingredients 

 of which are known, we can find out what the plant requires for its 

 growth. This method is termed water culture. 



The Food of Plants. Plants can only take in their food in the form 

 of compounds and in solution. 



Carbon is obtained by green plants from the carbon dioxide of the 

 atmosphere. Only the green parts of plants can decompose carbon 

 dioxide. Carbon dioxide passes into the plant through the stomata. 



Assimilation. The absorption of carbon dioxide and its conversion 

 into organic compounds is called assimilation. The conditions neces- 

 sary for assimilation to take place are (l) A certain intensity of light ; 

 (2) A certain temperature. The red parts of white light are the most 

 active in assimilation. 



Parasites and Saprophytes. Plants destitute of chlorophyll take in 

 'their carbon in the form of carbon compounds other than carbon dioxide. 

 Those plants which live on decomposing matter are called saprophytes. 



The Hydrogen necessary for a plant is obtained from water and 

 ammonium salts. 



Oxygen is required by a plant (a) in a combined form as a food, and 

 ''(b) in a free state for respiration. 



Respiration. All plants must have free oxygen for respiration. 

 This oxygen unites with the carbon of the plant and forms carbon 

 dioxide, which is given out by the stomata. Respiration and assimil- 

 ation are two different processes ; the plant gains by the first the energy 

 necessary for assimilation and growth, by the latter it gains weight. 



