As new molecules are formed, by mutual combinations of the food-materials selected by molecular attractions, they are taken up into the protoplasmic framework, and built in between those already in existence, thus distending the whole, and we say that the protoplasm _Assimilates_ food-materials and _Grows_. When distended beyond a given degree, or disturbed in various other ways, the molecular framework breaks, and some of the molecules are shattered, and as they fall to pieces certain of their constituent parts containing carbon and hydrogen forcibly combine at the moment of liberation with the oxygen in the fluid around and are burnt off in the form of carbon-dioxide and water, heat being of course evolved. This is the fundamental process of _Respiration_. It is probably the alternation of these processes of _Assimilation_--the building up into the protoplasmic structure of new complex labile molecules--and _Destruction_--the shattering of such molecules with redistribution, oxidation, etc., of their fragments--which constitute the fundamental process of life. Different authorities attempt to explain the details of these processes in various ways, but there is practical agreement on the one point, that life consists in the alternate building up of new protoplasm from the food-materials--_Assimilation_--and the breaking down of the molecular complexes to simpler ones--_Disintegration_, or _Dis-assimilation_, as we may call it. During the periods when assimilation prevails, and the protoplasm increases in mass, we recognise _Growth_, and since this is usually associated with the vigorous imbibition of water, owing to the powerful osmotic attractions for that liquid exhibited by some of the products, and with consequent further stretching of the invisible molecular plexus, the growth may be so evident in increased size, that we are accustomed to look upon the visible increase in volume alone as growth; but it is essential to understand that growth of the protoplasm is always proceeding during life, even when as many older molecules are being shattered and dispersed as new ones are being formed by assimilation, and when, therefore, no visible permanent enlargement occurs. Similarly, during periods when disintegration of the molecules prevails, we must not assume that the assimilation of new molecules is not occurring and that growth is not proceeding. The two processes are always going on during the active life of the protoplasm: in fact life consists in the play of these processes, as already said. That numerous chemical rearrangements of the atom-complexes take place outside the protoplasmic molecules--both of those left unemployed in assimilation and of those rejected during the destructive processes--will be readily understood: many of the bye-products found in plants, such as vegetable acids, alkaloids, colouring matters, crystalline bodies, etc., etc., are due to these, so to speak, fortuitous combinations and re-combinations.