344 Development and [BOOK n.

however, could now be explained quite simply after Schacht's discovery ; the formation of thylosis in vessels only takes place when these border on closed parenchyma-cells in the wood ; when this is the case, the very thin membrane which separates the bordered pits from the contiguous cells is not absorbed, but it bulges inwards into the cavity of the vessel under the pressure of the sap of the neighbouring parenchyma-cell, there swells up like a bladder, and may by the formation of partition- walls give rise to parenchymatous tissue ; this, if proceeding from a number of pits, fills up the cavity of the vessel.

3. HISTORY OF DEVELOPMENT AND CLASSIFICATION

OF TISSUES.

It has been already stated, that the first step to a real under- standing 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 ob- served, 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, intro- duced a terminology founded on real investigation, but did not succeed in bringing the subject to an entirely satisfactory con- clusion ; this could in fact be reached only by the study of the history of development, the only decisive method of investiga- tion, whether the object be to determine the true nature of cells and their subordinate forms, or the solid fabric of vege- table 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