178 ELEMENTS OF STRUCTURAL 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 obt;iinr.d by the 

 decomposition of water, and nitrogen from the ami'. 

 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 thesv. constituents of the dry plant, carbon is 

 the most abundant, 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 m chlorophyll; 

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

 These facts are proved by causing seeda to g "ow under 

 conditions which enable us to accuratsly coi'trol the 

 supply of nutrition in the form of carefully prepared 

 solutions of the different ingredients. Several sub 'tances 

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

 sodium, and some others, are in this way shown n\ v - to 

 be essential to healthy growth. 



309. The process by which the carbon, obtained f ror. > 

 the carbon dioxide, is combined with the elements 6\ 

 water to form starch is called assimilation. As al reach 

 explained, the particles of starch which are formed by 

 the chlorophyll granules in sunlight are converted by 

 combination with oxygen into soluble forms, and c 

 away, when the light is withdrawn, to other parts where 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



