This red oxide of iron, of which we have been speaking, is called by chemists the _peroxide_. There is another compound which contains less oxygen than this, and is called the _protoxide of iron_, which is poisonous to plants. When it exists in the soil it is necessary to use such means of cultivation as shall expose it to the atmosphere and allow it to take up more oxygen and become the peroxide. The black scales which fly from hot iron when struck by the blacksmith's hammer are protoxide of iron. The _peroxide of iron_ is a very good absorbent of ammonia, and consequently, as will be hereafter described, adds to the fertility of the soil. [What can you say of the oxide of manganese? How do you classify the inorganic constituents?] OXIDE OF MANGANESE, though often found in small quantities in the ashes of cultivated plants, cannot be considered indispensable. Having now examined all of the materials from which the ashes of plants are formed,[F] we are enabled to classify them in a simple manner, so that they may be recollected. They are as follows:-- ALKALIES. ACIDS. NEUTRALS. Potash. Sulphuric acid. Silica. Soda. Phosphoric " Chlorine. Lime. Oxide of Iron. Magnesia. " Manganese. FOOTNOTES: [B] Bromine, iodine, etc., are sometimes detected in particular plants, but need not occupy the attention of the farmer. [C] This classification is not strictly scientific, but it is one which the learner will find it well to adopt. These bodies are called neutrals because they have no decided alkaline or acid character. [D] In some soils the _fluorides_ undoubtedly supply plants with soluble silicates, as _fluoric acid_ has the power of dissolving silica. Thus, in Derbyshire (England), where the soil is supplied with fluoric acid, grain is said never to lodge. [E] Sourness. [F] There is reason to suppose that _alumina_ is an essential constituent of many plants. CHAPTER V. GROWTH. [Of what does a perfect young plant consist? How must the food of plants be supplied? Can carbon and earthy matter be taken up at separate stages of growth, or must they both be supplied at once?] Having examined the materials of which plants are made, it becomes necessary to discover how they are put together in the process of growth. Let us therefore suppose a young wheat-plant for instance to be in condition to commence independent growth. It consists of roots which are located in the soil; leaves which are spread in the air, and a stem which connects the roots and leaves. This stem contains sap vessels (or tubes) which extend from the ends of the roots to the surfaces of the leaves, thus affording a passage for the sap, and consequently allowing the matters taken up to be distributed throughout the plant. [What seems to be nature's law with regard to this? What is the similarity between making a cart and raising a crop?