Plant body is made of material units, cells. Each cell has protoplasm usually surrounded by a rigid wall. Protoplasm undergoes metabolic changes and produces many ergastic substances. As plants cannot take solid food, they manufacture complex organic matters out of simple substances. We know that the whole animal kingdom, including human beings, has to depend on plants for the supply of food.
Chemically, protoplasm is a very complex matter having proteids, carbohydrates, fats, and other inorganic substances in composition. Proteids are made of carbon, hydrogen, oxygen, nitrogen, and in addition, often sulphur and phosphorus; carbohydrates and fats are composed of carbon, hydrogen and oxygen. Cell wall is primarily made of insoluble carbohydrate, cellulose.
An idea about the chemical composition of the plant body can be had by the following method. A plant is dried at a high temperature. Naturally it will lose water. The percentage of water in plants is very high, often as much as 95%. The proportion of water can be determined by proper weighing. The dried plant gets charred forming a black mass of charcoal.
That is mainly carbon which forms almost half the dry weight of the plant. Now it is burnt. The combustible matters like proteins, carbohydrates and fats are converted into carbon dioxide, sulphur dioxide, ammonia, water vapour, and other gases and thus escape, which may be properly collected and composition noted.
The white incombustible residue left behind is ash, which is nothing but the oxides of the metals constituting the plant body. The percentage of ash varies; but on the average, it forms nearly 5 per cent, of the total weight of the plant. On proper analysis ash is found to be composed of quite a large number of elements, many of them, of course, being present only in minute traces.
The following elements are, however, always present in ash: potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), sodium (Na), sulphur (S), phosphorus (P), chlorine (CI), and silicon (Si). Elements like manganese (Mn), boron (B), zinc (Zn), copper (Cu), and molybdenum (Mo) may be present in slight traces.
