TFIE VEGETABLE CELL. 31 



foce of the cross section of tlie primary membrane appeared as so narrow 

 a line, tliat it was taken for the boimdary line between two neighboimng 

 cells, and was drawn as such. Subbe(][iiently, when the knowledge of 

 cellulose structure had progressed farther, the primaiy membrane was 

 distinguished from the secondary layers, and the outermost layer of cell- 

 membrane was seen under stronger microscopes to have a clearly visible 

 breadth (or thickness), the idea remained of an easily distinguishable 

 boundary line between the two coherent cells, and such a line was even 

 figured. This, as Hartig correctly observed, was untrue, for om* micro- 

 scopes, do not shew any boundary line between the two coherent piimary 

 membranes (see figs. 21, 22, 2i5, 32, 44). When Hartig drew from this 

 the general conclusion that no limit exists, and that the outer membrane 

 of the two cells is common to both, his induction was too hasty. The 

 impossibility of seeing a line of demarcation with our microscopes, war- 

 i^ants, ct priori, nothing more than the conjecture that our present instru- 

 ments are not yet sufficiently perfect for the purpose. It is self-evident 

 under these circumstances that nothing has been accurately made out as 

 to the manner in which cells are connected. 



The cells cobering together may be separated from eaci other; 

 in very succulent tissues, as in the parenchyma of many juicy 

 fruits, a slight pressure suffices for this ; in somewhat firmer tis- 

 sues the connexion of the cells may often be so loosened by boil- 

 ing in water or by freezing, as to become easily separable ; while 

 in very solid tissues a long maceration in water or a short boiling 

 in nitric acid is necessary. It might be imagined that the double 

 nature of the outer membrane could be readily demonstrated by 

 this separation of the cells, but wrongly, for I found that the 

 outer membrane, when distinctly perceptible, was not split into 

 two layers in such cases, but torn into pieces, some adhering to 

 one and some to the other cell, so that the separated cells were 

 composed chiefly of secondary layers.* 



It has been remarked already, in the description of the form of 

 cells, that the flat faces of cells meet at sharp angles in compara- 

 tively few cases, since the corners and edges are generally rounded 

 off. It follows necessarily from this condition, that the cells are 

 not, for the most part, coherent together by their whole surfaces, 

 but leave empty spaces between them, which run along the edges 

 of the cells in the form of triangular canals having no special 

 walls of their own, opening into each other at the corners of the 

 cells, and so forming a net- work of narrow and wide tubes branch- 

 ing throughout the whole plant, to which the name of intercellu- 

 lar 'passages has been applied {see figs. 6, 7). In living plants they 

 are, with few exceptions, filled with air. 



* Schultz has lately made known a process for isolating the conjoined 

 cells even of woody tissues. It consists in boHing them for a short time 

 with chlorate of potash in nitiic acid. It is not clear, however, that thm 

 does not dissolve the outer membranes. — A. II. 



