THE FORM OF THE PLANT CELL 13 



division, implying that the cells are in fact not all of the same size, and 

 in any case periodically reducing the number of faces by the insertion 

 of a new face diametrically across such a 14-sided polyhedron, will 

 assure some divergence in shape; but nevertheless it must be borne in 

 mind that cells do approximate to this shape or to some derivative of it. 

 Obviously any derivative of the orthic tetrakaidecahedron produced 

 by parallel displacements will equally well fill space and, in particular, 

 the polyhedron can be elongated in any direction without losing its 



Fig. 2 (a) 



Fig. 2. Diagrammatic representation of a cell of the 

 apical meristem and a cell produced from it by 

 elongation of four hexagonal and two tetragonal faces 

 in the same zone. Note that both cells have an axis 

 of twofold symmetry parallel to the length of the 

 page. For convenience the isodiametric and the 

 elongated cell are drawn in the same orientation; it 

 does not necessarily follow that only cells oriented as 

 in Fig. 2{a) could give cells as in Fig. 2{b), since elon- 

 gated cells will take up the form illustrated in the 

 latter figure, on account of the presence of neighbour- 

 ing cells, irrespective of the orientation of the 

 parent cell. 



Fig. 2(6) 



space-filling power. This is what in fact usually happens in the develop- 

 ment of fibrous cells and one possibihty, founded on the careful 

 observation of Lewis (2), is shown in Fig. 2. 



In the growing apices of roots and shoots, where cell division is 

 occurring rapidly, then, the cells will approximate to more or less 

 regular 14-sided polyhedra. In the development of stem and root, 

 however, some of the facets of some of these polyhedra begin to 

 elongate. It is not proposed here to examine at all the factors which 

 cause such cells to elongate rather than to swell up uniformly, but 

 merely for the moment to accept the fact of elongation itself. This 

 unidirectional expansion of six faces of the polyhedron (Fig. (2^)), with- 

 out change in their general form, imphes that four of the longitudinal 

 faces of an elongated cell are effectively hexagonal, and two tetragonal, 

 provided that the cell tissue still fills space completely. The cell as a 



