An apparently dead grain is placed in the soil. If the temperature is a few degrees above the freezing point, and the soil holds a due quantity of water, the integument of the seed imbibes moisture and swells; the tissue is softened, and the first effort of vital force begins. The seed has now the power of decomposing water, the oxygen combines with some of the carbon of the seed, and is expelled as carbonic acid. Saussure’s experiments prove this. The air above the soil in which a horse-bean was placed to germinate, gave, before the experiment, nitrogen 210·26, and oxygen 56·29, and after germination, nitrogen 209·41, oxygen 44·38, and carbonic acid 11·27. This part of the process is but little removed from the merely chemical changes which we have already considered. We find the starch of the seed changed into gum and sugar, which affords nutritive food for the developing embryo. The seed now lengthens downwards by the radicle, and upwards by the cotyledons, which, as they rise above the earth, acquire a green colour. Here the first stage of vegetable life ends, the chemically exciting process is at an end, and a new stimulus is required to continue in full activity the vital powers. Carbonic acid is no longer given off. The cotyledons, which are two opposite roundish leaves, act as the lungs; by them carbonic acid taken from the atmosphere is absorbed and carried by a circulating process, now in full activity, through the young plant. The carbonic acid, a compound of carbon and oxygen, is decomposed; it is deprived of its carbon, which is retained by the plant, and oxygen is exhaled. The plant at this period is little more than an arrangement of cellular tissue, a very slight development of vascular and fibrous tissue appearing as a cylinder lying in the centre of the sheath. At this point, however, we begin more distinctly to trace the operations of the new power; the impulses of life are strikingly evident. The young root is now lengthening, and absorbing from the moisture in the soil, which always contains some soluble salts, a portion of its nutriment, which is impelled upwards by a force--probably capillary attraction and endosmose action combined--to the point from which the plumule springs. Capillary force raises the fluids through the tubes in the stalk, and conveys them to the veins in the leaves, while the endosmose force diffuses them through the vegetable tissues. The plumule first ascends as a little twig, and, at the same time, by exerting a more energetic action on the carbonic acid than the cotyledons have done, the carbon retained by them being only so much as is necessary to form chlorophylle, or the green colouring matter of leaves, some wood is deposited in the centre of the radicle. From this time the process of lignification goes on through all the fabric,--the increase, and indeed the life, of the plant depending upon the development of a true leaf from the plumule.