CHAP, in.] of Cell-membrane in Plants. 301 



chief features of his view in 1828 in his first work, ' Die Poren 

 des Pflanzengewebes ! ' The way in which he represented to him- 

 self the growth in thickness of cell-membranes at a later time 

 may be expressed as follows. All elementary organs of a plant 

 are originally very thin-walled perfectly closed cells, which in 

 the tissue are separated by walls formed of two laminae : ; on 

 the inside of these primary cell-membranes, after they have 

 ceased to increase in circumference, new layers of membranous 

 substance are formed, which lying one upon another adhere 

 closely together, and represent the whole amount of secondary 

 thickening layers ; on the inner side of the membrane thus 

 thickened by apposition there may usually 2 be perceived a 

 tertiary layer of a different character. 



But there are certain sharply defined spots on the original 

 cell-wall, where this thickening does not take place ; in such 

 spots the cell is still bounded only by the primary membrane ; 

 it is these thin spots which bear the name of pits, and which 

 Mirbel, and in some cases Moldenhawer, took for holes, but 

 von Mohl considered that it was only in very exceptional cases 

 that they were really changed into holes by resorption of the 

 thin primary wall. In accordance with this theory, the spiral, 

 annular, and reticulated vessels are produced by deposition of 

 thickening matter in the form suitable to each case on the 

 inside of the originally smooth thin cell-wall. But like Schlei- 

 den and other phytotomists, von Mohl was not quite clear in his 

 views either of the origin or mode of formation of matured 

 bordered pits ; it was supposed that the two laminae of the 

 dividing wall parted from one another at certain spots in such 

 a manner that a lenticular hollow space was formed between 

 them, and that this space answered to the outer border of the 



1 But von Mohl expressed some doubts on this point in 1844 ('Botanische 

 Zeitung,' p. 340). 



1 This tertiary layer was at first supposed by Theodor Hartig to be of 

 general occurrence; von Mohl in 1844 considered it to be present only in 

 certain cases. 



