THE VEGETABLE UELL. 



11 



F((! IS. 



degree of concentration the membrane is caused to swell up, ifcs 

 lamellar structure becomes very mucli more distinct, and a great 

 number (often fifty) of separate layers may be detected. By this 

 means the lamellar stiuctiire may be demonstrated even in those 

 cases Id which the unaltered membrane appeared completely homo- 

 geneous ; for instance, in the horny cells of the albumen of Fhy- 

 tdeplias. Usually the wall of the cell is of equal thickness on 

 all sides ; in this case the layers run uninterruptedly round the 

 cavity and form perfect cells encased one within another. In 

 many cases (e, jr., very fi^cquently in the epidennis-cells — fig. 1 3 — 

 and in the brown cells which 

 surround the vascular bundles 

 of the Ferns) the different sides 

 of the cell possess, on the con- 

 trary, a very different thick- 

 ness ; in this case the layers of 

 the thicker portion of the wall 

 are not continued over the thin 

 sides, but are bevelled gradu- 

 ally oflf! 



This condition alone allows 

 us to conclude with great pro- 

 bability, that the growth of 

 the cell-membrane in thickness 

 does not depend upon the thin 

 mexnbrane of the vouns cell it- 

 self growing thicker by the absorption of new celMo.e, bnt that 

 it arises from a periodical deposition of new membranes upon the 

 already completely developed wall. But the complete confiimation 

 and more accurate knowledge of this process are only obtained 

 through the circumstances next to be mentioned. 



The wall of young cells having yet very thin membranes, ap- 

 pears perfectly smooth and uniform ; but if the tissue of the same 

 organ is examined at a later period, the walls of its cells are found 

 to have become thickened ; these walls are almost without excep- 

 tion found to be covered with a greater or smaller number of pore- 

 like points or slits, which are distinguished by the name of dots 

 (tiipfel ox pits). A more minute examination of the cross-section of 

 the cells (figs. 11, 12) reveals that these spots are formed by canals 

 which open freely into the cavity of the cell, but are closed externally 

 by the outermost thin membrane of the cell "When all these cir- 

 cumstances are taken together, it becomes most indubitably evi- 

 dent, that the primary membrane of the cell is completely closed 

 and not possessed of visible pores ; that the subsequent deposits, 

 on the contrary, have the form of perforated membranes, and 

 that the deposition of these secondary membranes takes place in 

 the direction from without inwards upon the inside of the primaiy 

 membrane. 



Cells of tlie Epideniiis of the stem of Yuetmi album. 



