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 



