CHAPTER V. 



WATER ABSORPTION AND TRANSPIRATION. 



137. The Pood Materials of a Green Plant. If we 

 make a chemical analysis of a plant an analysis of the 

 gases given off and the residue or ash left behind on burning 

 the plant we find the following chemical elements : carbon, 

 oxygen, hydrogen, nitrogen, sulphur, phosphorus, calcium, 

 magnesium, potassium, iron, sodium, silicon, and chlorine ; 

 traces of other elements are sometimes present. Of all these 

 elements, only the first six actually enter into the composition 

 of the living substance (protoplasm) of the plant. It is evident 

 that all the elements found in the plant must enter it in the 

 food materials absorbed. All the carbon required by an 

 ordinary green plant comes from the carbon dioxide of the 

 air, which is absorbed by the green parts of the plant, while 

 the other elements are absorbed by the roots, usually in the 

 form of nitrates, sulphates, phosphates, chlorides, and silicates 

 of potassium, sodium, calcium, magnesium, iron, etc. 



138. Water, Carbon, and Ash in Plants. In several 

 of our experiments we have seen that water is present in 

 fresh plant-bodies, even in " air-dry " parts like seeds as sold 

 in shops, and that the water can be driven off by drying 

 plants in an oven without charring them. In woody parts 

 the percentage weight of water is about 50, in herbaceous 

 parts (e.g. fresh leaves or whole seedlings) about 75, in fruits 

 and in fleshy plants (e.g. Stonecrop) from 85 to 95, and in 

 water-plants and Algae from 95 to 98. The residue in each 

 case the dried substance gives the " dry weight." 



When the dried parts are burnt, the organic substances 

 of which they largely consist are converted into inorganic 

 substances, chiefly carbon dioxide and water, which escape 



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