For the decomposition of carbonic acid and water, we find that light is required; that where there is a deficiency of light this action goes on put partially. Researches have proved that while the blue rays are most active in germination, the yellow rays act more readily on the developed plant. Mr. Robert Hunt, in a lecture which he delivered at the Royal Institution last year, on "Light and Actinism," stated some valuable and curious facts. He considers germination to be entirely dependent upon the actinic, but to be actually impeded by the luminous rays; while on the other hand, this decomposition of carbonic acid, this lignification is most extensively carried on by the action of the luminous power, and is stopped by the actinic force. As summer advances, the thermic and the pa-rathermic rays are roost conducive both to fruiting and flowering. All that we can say to these carefully investigated and well proved facts, is, that they give us one of the most striking examples of the adaption of inorganic nature to organic life, that can be found in the whole range of physical science.

From the first moment of the germination of a seed, carbonic acid is always being absorbed, but not always, as I have before mentioned, being decomposed; for in the dark this action is stopped, but the carbonic acid still continues to be absorbed by the juices which the plant holds in solution. This action was very aptly compared by Professor Lie-big - this emission of water and carbonic acid from a plant in the dark - "to a cotton wick enclosed in a lamp containing a liquid saturated with carbonic acid." Water and carbonic acid are taken up by the wick by capillary attraction, both evaporating on its exterior surface. In the night another action goes on in the growth of plants - the absorption of oxygen; an action as purely chemical, as the evolution of carbonic acid was purely mechanical. Yet, because they occur simultaneously, it was presumed that they were subject to the like causes; even after it was found that their ratios of action were not equal; for plants absorb more oxygen than they emit carbonic acid.

This nightly absorbtion of carbonic acid is, to a certain extent, independent of the life of the plant, not acting upon the main parts, but upon the blossoms, fruit and leaves, and the result of experiment has revealed to us the facts, that leaves containing highly nitro-genised compounds, or volatile oils, absorb oxygen more vigorously than leaves which conof oxygen, is converted into a resin. The Agave americana, absorbs 0.3 times its volume of oxygen in twenty-four hours; the Pinus abies, containing volatile and resinous oils, ten times its volume of oxygen in the same time; the Quercus robur, containing tannic acid, fourteen times its volume of oxygen; and the Populus alba, twenty-one times its volume of oxygen during a day and night.

I need hardly mention as a familiar example of these chemical changes, caused by the absorption of oxygen, Cacalia ficoides, which is sour in the morning, tasteless at noon, and bitter in the evening from the excess of hydrogen; it became tasteless when there was no excess of oxygen, and sour owing to the oxygen which it had absorbed during the night.

This decomposition of carbonic acid is most interesting to us as exhibiting clearly the real process of lignification; as helping to establish correct notions regarding that vegetable matter undergoingeremacausis which is familiarly known as humus; experiments have proved the insolubility of humus; calculations have demonstrated, that suppose there existed a superabundance of the most soluble salt of humic acid, still all the carbon which it might contain, would be totally inadequate to give us but a very small portion of that carbon which is found in vegetation. But another calculation was made as to a different source for the carbonic acid; a calculation which was based upon De Saussure's accurate determination of the amount of carbonic acid present in the atmosphere, an amount of a little more than one-thousandth of its weight; yet we find that the air contains no less sum than 3085 billions of pounds of carbon, a quantity surpassing in weight not only the carbon of existing vegetation, but also that which is at present locked up in the mineral coal which is distributed over certain parts of the earth's surface.

It may now well be asked, How lives the young plant before it comes in contact with the atmosphere, the source of its carbonic acid? The reply to this question is the key to the proper action action of humus. This humus is especially useful for the support of young plants; it takes oxygen from the air; and then furnishes the plant with carbonic acid - this is its great use. We see, then, that the process of nourishment in a young plant totally differs from those actions by which the well developed vegetable is supported. A young plant causes a certain quantity of oxygen to be abstracted from the atmosphere, while an old one furnishes us with a never-failing source of this gas.

I cannot help referring to the fact, that many juicy and milky plants in warm countries, flourish on a soil destitute of humus, containing absolutely not a trace of carbon; and sometimes are found being held by one point of attachment to this barren soil - such shrubs as the Cactus and the Caoutchouc are among this number. Baron Humboldt especially mentions the Cactus tribe. In his beautiful paper on the "Physiognomy of Plants," he states: - "In the waterless plains of South America, animals suffering from thirst seek the Melon-Cactus, a spherical plant half buried in the dry sand, and encased in formidable prickles, but of which the interior abounds in refreshing juice. The stems of the columnar Cactus often rise to the height of from thirty to thirty-two feet; they are often covered with lichens, and dividing into candelabra-like handles, resemble in physiognomy some of the Euphorbias of Africa."Again, in note 20: - "When one has been accustomed to see Cactuses only in our hot-houses, one is astonished at the degree of density and hardness which the ligneous fibres attain in old Cactus stems. The Indians know the Cactus wood is incorruptible and excellent for bars and the thresholds of doors.