CELLULAR TISSUE. Ol 



without it in some form or other, and many being entirely com- 

 posed of it, is that called cellular tissue, from its being made up 

 of a number of separate cells, or minute bags, adherent together. 

 These, when first formed, are usually nearly globular, or of a 

 figure resembling an 

 egg; so that, if cut 

 across, their walls would 

 exhibit a series of cir- 

 cles touching each other 



rninf (V\cr 

 points {I Ig. 



FIG. 21. VARIOUS FORMS OF CELLULAR TISSUE ; a, 

 geparate yesicles Qf ^ egg . shape d form ; b, section of 

 Afterwards, cubical cellular tissue of pith; c, section of muriform 

 cellular tissue. 



OOOoor 



at 



21, ). 



, , , 



however, they are gra- 

 dually pressed against each other, and their sides become flat- 

 tened. Their form will then depend upon the amount of the 

 pressure on the respective sides. If it have been equal in all 

 directions, the cell will sometimes be cubical, as it is often found 

 in pith (Fig. 21, &) ; or it will have the form termed the dode- 

 cahedron, which is a solid having twelve 

 equal sides (Fig. 22). But if it be pressed 

 more on one side than another, it will be 

 narrowed in that direction, and elongated 

 in the other. Thus the original form of 

 the cell may become greatly modified, 

 during the growth of the plant. In gene- 

 ral, the greatest elongation take,s place 

 in the direction of most rapid increase ; 



FIG. 22. SECTION OF IRREGU- , , . . , , , - . . , 



LARLY COMPRESSED CELLULAR but this is not always the case ; for in the 

 TISSUE ; the honey-comb ap- gtems of most trees in this climate, there 



pearance of the greater part . 7 



is due to the 12-sided form of is a peculiar set of cells extending from 



SSXZSSZSSi tte .P ith towards th f b^.^ich have 



tion, present hexagons or their greatest length in a horizontal direc- 

 tion ; and the cells being of an oblong 



flattened form, and arranged like bricks in a wall, this kind of 

 structure has been called muriform (wall-like) cellular tissue 

 (Fig. 21, c). 



72. From what has been said of the permeability, or power 

 of giving passage to fluids, which vegetable membrane possesses, 





