This hypothesis of formative stuffs held by such able botanists as Sachs and Goebel demands nevertheless serious consideration, if for no other reason than that if it is true it offers quite a simple explanation of many phenomena of growth and of regeneration. We should, I think, distinguish between specific or formative stuffs and building or food stuffs. By specific stuffs is meant a special kind of substance which, being present in a part, determines the nature of the part. Sachs supposes, for instance, that a specific substance is made by the leaves of a plant which is transported to the vegetative, growing region (which has so far produced only leaves), and changes its growth so that flowers are produced. Goebel does not commit himself altogether to specific stuffs of this sort, but speaks also of building stuffs. By building stuff we may understand food material that is necessary for growth, and from which any part of the plant may be made. Its presence in larger or smaller quantities may determine what a particular part shall become, but further than this it exerts no specific action. This means that the presence of a certain amount of food substance may determine what a given region shall produce, but it is not supposed that there are different kinds of food materials that correspond to each kind of structure. If there were such, they would not differ from specific substances, unless we wish to make subtle distinctions without any basis of fact to go upon. Goebel points out that there is evidence to show that the greater or less quantity of food substance contained in a plant often determines the nature of its growth, as for instance the production of flowers when the food supply runs low and the production of foliage when the food supply is abundant. This difference may explain Sachs’ experiment with begonia leaves; and if so, there is no need for supposing specific flower stuffs to be made in the plant. There is another point of view which has been, I think, too much neglected, viz. that the production of food stuffs is itself an expression of changes taking place in the living tissues, and if the structure is changed so that it no longer produces the same substances it may then lead to the development of different kinds of organs. The difference in the regeneration of an apical and a basal leaf of begonia may be due to some difference in the structure of the protoplasm. The greater or smaller amount of starch produced in these leaves may be only a measure of, and not a factor in, the result.