546 lewis. 



equal the hexagons in vertical measurement. A cell (Figure 2, a) with 

 less of an equatorial ridge than that which has just been described, is 

 shown in Figure 7. As seen by comparison with Figure 3, six of the 

 seven surfaces which appear in the drawing are exactly according to 

 the type. 



A prolific cause of deviations from the tetrakaidecahedral form is 

 found in the process of cell-division ; and that this must be so, is evi- 

 dent upon reflecting that the division of a typical cell, whether vertical 

 or transverse, reduces the number of surfaces from fourteen to eleven. 

 Figure 1 1 is the pattern of vertical division through angles of the top 

 and bottom hexagons, but the models afford no evidence that division 

 in this plane actually takes place; Figure 12 shows vertical division 

 through the sides of these hexagons; and Figure 9 illustrates trans- 

 verse or horizontal division. With the completion of the process, a 

 constriction occurs along the plane of division, whereby the daughter 

 cells become more globular. In all cases the resulting cells have but 

 eleven surfaces. Restoration of those which are lacking will depend 

 upon the division of adjacent cells. 



In elder-pith growth is chiefly in length, and division is predomi- 

 nantly transverse. The orientation of the cells, as will be seen, is in 

 accordance with this. If the growth were chiefly in thickness, as in 

 cork, it would be expected that the cells would be so turned that their 

 inner and outer surfaces, instead of their upper and lower surfaces, 

 would be hexagonal. In a typical cell oriented as in pith the trans- 

 verse plane passes through hexagonal surfaces only, being above or 

 below the quadrilateral surfaces in every case (Figure 9). If the cells 

 in contact with the one which has thus divided, likewise divide trans- 

 versely in the middle, the planes of their division will encounter the 

 surfaces of the central cell as shown by the dotted lines in Figure 9. 

 The position of these lines may perhaps be better understood by 

 dividing several cells in the group shown in Figure 3 transversely 

 through the middle. A line will pass across every lateral hexagonal 

 surface, from square to square, bisecting the sides of the squares ; and 

 they cut off from each hexagon a small quadrilateral area, leaving the 

 remaining portion still hexagonal. The contraction which follows the 

 division of the adjacent cells produces ridges along the lines passing 

 from square to square, and the tension or pull upon these squares con- 

 verts them into hexagons. The diagram, Figure 8, shows, then, the 

 restoration of the tetrakaidecahedral form, following the transverse 

 division of the cell shown in Figure 9 and of those in contact with it, 

 the corresponding surfaces in the two drawings being marked with the 



