Assimilation, a term now used in vegetable physiology in a somewhat narrow sense, viz. for the retention of the carbon of atmospheric carbonic acid gas by green plants under the influence of light. The first stage in this process seems to be the union of the carbon dioxide with water taken in by the roots to form some polymer of formic aldehyde such as glucose in solution, starch being the first visible result. In animal physiology the term assimilation is used in a wider sense to include the whole of anabolism (q.v.), i.e. all the constructive changes in the food-substances after their first mere taking in, though most of these substances will be already not only organic but organised.