It has been already stated, that the first step to a real understanding of the structure as a whole of the higher plants was made by Moldenhawer, who beginning with the study of the Monocotyledons, first formed an idea of the vascular bundles as a distinct whole, a system composed of various forms of tissue, and applied this idea to explain the construction of the stems of Dicotyledons, upsetting thereby Malpighi’s earlier theory of the growth in thickness of stems. It was also observed, that von Mohl, advancing further in the same direction, gave a more exact description of the epidermis and of the tissues connected with it, and classified them, that is, introduced a terminology founded on real investigation, but did not succeed in bringing the subject to an entirely satisfactory conclusion; this could in fact be reached only by the study of the history of development, the only decisive method of investigation, whether the object be to determine the true nature of cells and their subordinate forms, or the solid fabric of vegetable structure, or as in the present case to distinguish and classify forms of tissue; it is this method which supplies the morphological points of view necessary for the understanding of the inner structure of the plant by investigating tissues in those states of development, in which they are not yet adapted to subsequent physiological functions. The combination of morphological and physiological points of view in the determination of facts has maintained itself longer in this part of botanical study than in any other; but here too ideas and opinions were gradually sifted and cleared up under the influence of the modern study of the history of development, though it was not till after 1850 that the determination of the chief points in the theory of cell-formation left the leading phytotomists at liberty to devote themselves to histological questions.