CH. x THE PHYSIOLOGY OF NUTRITION 117 



of its food are spoken of as nutrition. The nourishment of the 

 plant can only go on where food materials are taken in and so 

 changed that they can become a part of the plant. If the food- 

 supply is not kept up the death of the plant is a foregone 

 conclusion. Growth can only go on where the food-supply is in 

 excess of that demanded for the production of the energy 

 expended during its present activity. 



The Composition of Plants. The most abundant in- 

 gredient in a living plant is water. Many succulent plants, such 

 as Turnips and Cabbages, contain more than 90 per cent, of water. 

 Timber which is felled during the driest season of the year 

 seldom contains less than 40 per cent, of water. If a plant is 

 dried at a temperature of from 230 F. to 248 F. all water is 

 expelled and the solid matter alone remains. 



The solid matter of a plant can easily be made to burn, and 

 the greater part will disappear in the form of gas, a white ash 

 only being left behind. If the gases which are given off during 

 the combustion of the solid matter of a plant are collected and 

 examined, they are found to consist of carbon dioxide, water, 

 ammonia, and a compound of sulphur. If these compounds are 

 split up into their elements they are found to consist of Carbon, 

 Hydrogen, Oxygen, Nitrogen, and Sulphur. 



Without these five elements no plant can be produced, and 

 they are called the combustible elements of the plant because 

 they can be burnt off. Carbon may form as much as one-half 

 of the dried substance of a plant. The nitrogen seldom exceeds 

 4 per cent, of the dry matter, and in far the larger number of 

 cases is present in much smaller quantities, while the amount 

 of sulphur present is still smaller. The remaining fraction 

 of the solid part of plants is composed of oxygen and hydrogen, 

 and a little mineral matter. 



The ash of the plant is found, when analysed, to contain 

 Phosphorus, Potassium, Calcium, Magnesium, and 

 Iron, along with Silicon, Sodium, Chlorine, and slight 

 traces of most other chemical elements. Silicon, Sodium, and 

 Chlorine, are not necessary for the growth of the plant, but are 

 taken in along with the water. 



Chlorine seems to be necessary for the nutrition of Buck- 

 wheat, Barley, and Oats, for if these plants are grown in 

 solutions which do not contain this element they do not flourish. 



