178 ELEMENTS OF STEUCTUEAL BOTANY. 



form starch for purposes of growth. The oxygen 

 required by the plant is derived chiefly from the carbonic 

 acid gas and from water. Hydrogen is obtained by the 

 decomposition of water, and nitrogen from the ammonia, 

 which, like the carbon dioxide, is carried down from the 

 air by rain, and also from nitrates contained in the soil. 

 Sulphur is obtained from salts (such as calcic sulphate) 

 found in the soil, as are also, of course, all the inorganic 

 elements. 



Of all these constituents of the di-y plant, carbon is 

 the mostabundant, amounting to about half of the entire 

 weight. 



308. The inorganic elements, though small in quantity, 

 are, nevertheless, essential. If, for' example, a plant be 

 altogether deprived of iron it will produce no chlorophyll; 

 while, if potassium is withheld, it will not produce sjtarch. 

 These facts are proved by causing seeds to grow under 

 conditions which enable us to accurately control the 

 supply of nutrition in the form of carefully prepared 

 solutions of the different ingredients. Several substances 

 of common occurrence in the ash of plants, as silica, 

 sodium, and some others, are in this way shown not to 

 be essential to healthy growth. 



309. The process by which the carbon, obtained from 

 the carbon dioxide, is combined with the elements of 

 water to form starch is called assimilation. As already 

 explained, the particles of starch which are formed by 

 the chlorophyll granules in sunlight are converted by 

 combination with oxygen into soluble forms, and carried 

 away, when the light is withdrawn, to other parts where 

 growth is going on, or to storehouses such as tubers and 

 seeds. This oxidising and converting process is metastasis. 



