This power of decomposing carbon dioxide (carbonic acid), and of decomposing ammonia into hydrogen and nitrogen, and water into hydrogen and oxygen, and of recombining, solidifying, and utilising these elements in their structure, is possessed by all plants containing chlorophyll in their leaves and in the lower forms of plant-life, as Paranema in their free cells. The cells by which plants absorb oxygen and disengage carbon dioxide are not cells containing chlorophyll.

The results of the recent investigations of Mr H. C. Sorby of Sheffield, tend to show that the colouring matter in the green leaves of plants are not simple green-coloured granules, but mixtures of chlorophyll and other colouring substances in a free state, differing in plants, and having different chemical reactions, requiring new names, as chlorophyll, blue and yellow chlorophyll, xanthophyll, orange and yellow xanthophyll, lichnoxanthine, and orange lichnoxanthine, etc.; and that the word endochrome would be a better word to use than chlorophyll as a general term. Further researches in this branch of science, called comparative vegetable chromatology, may tend to explain the causes of many of our beautiful autumnal tints, and of the presence of the colouring substances in the petals of the corolla.

It has been stated, generally, that plants with chlorophyll in their leaves have the power of decomposing and recombining the elements carbon, hydrogen, oxygen, and nitrogen into various states or compounds, but it is not correct to suppose that they are all equally endowed with the extraordinary powers of recombining and rearranging some or other of these elements into all of the following compounds. It will be easily seen, from the names of some of them in the third division, that they are confined to certain plants, and not possessed by other plants. The following are the compounds alluded to: Firstly, compounds of the elements, carbon, hydrogen, and oxygen, generally called ternary, sometimes carbonaceous compounds, or carbo-hydrates, and to these elements the plant is indebted for its cellulose (the basis of all vegetable tissue), starch, dextrine, sugar, and fixed oils; secondly, compounds of the elements, carbon, hydrogen, oxygen, and nitrogen, generally called quaternary, sometimes nitrogenous compounds, and to these elements the plant is indebted for its albumen, fibrine, diastase, and caseine; and thirdly, to all the four elements above named the plant is indebted for its acids, such as citric, acetic, tartaric, malic, tannic, hydrocyanic, oxalic, and gallic acids; and for its alkaloids, such as aconitine, atropine, quinine, morphine, strychnine, theine, or caffeine; and for its volatile oils, resins (copal); and for the hydro-carbons, caoutchouc and gutta-percha. It may be here mentioned that the milky secretions of our common wild-flowers, such as of the Dent de lion or Dandelion, Sow thistle, (Sonchus crispus), Spurges (Euphorbia), or Celandine (Chelidonium majus), may be regarded as caoutchouc.

Leaves, by exhalation or transpiration, give off in vapour the water of the sap: sun-heat and wind are both causes of this natural process, and a fresh supply of water will be necessary to restore the equilibrium; but it does not go on so rapidly during dull quiet days, and the gardener therefore does not then water his plants so freely as might be otherwise necessary to do after they have been exhausted by sun-heat, nor does he water his plants during sun-heat, nor in windy weather, as it would only tend to increase the exhalation or transpiration, which, when excessive, carries off so much heat from the plant. It has been said that one of the uses of garden-walls is to keep off the wind, and so lessen the exhalation of plants. Du Breuil says that watering fruit-trees growing in the open air, except during their first year of planting, does more harm than good, and to stone fruit-trees is destructive to their roots.

Some tuberous-rooted plants may be propagated by their leaves without buds on them, as Gloxinia gesnera, tuberous Begonia, and many succulent plants. The process by which this is induced is very simple, its direct cause more difficult to explain. The leaf is detached from its parent plant, placed flat on the surface of a pot containing silver-sand, and the veins are cut or broken, and an artificial node or axil is produced; the leaf is pegged down, or kept down in its place, and with the usual appliances of heat, warmth, moisture, and a bell-glass over it, it soon emits roots which will form small bulbs, even at the edges of the leaf, and the green fleshy part of the leaf will perish.

Leaf-buds consisting of wrapped-up leaves are formed, in temperate and cold climates, at the end of the stem or branch, in the autumn, or in the axils on the stem, where the leaves or leaf-stalks are situate;* the mode or plan on which the bud is formed is called "vernation." These buds or eyes, as they are called, can be easily cut out from the stem of a plant, such as the Vine, and be induced to grow in warmth; or they can be cut out from one tree and placed in the stem of another tree when the sap is flowing - and to this faculty is due the practice of budding fruit-trees, as well as Rose-trees on the Briar, and which will to some extent supersede the old system of grafting.

* Trees and shrubs in hot climates do not produce buds; the interval between the formation and evolution of the bud is so short as not to need this protection for the shoot.