640 THE FORMS OF TISSUES [ch. 



an epidermal layer, whose cells subdivide further by partitions 

 perpendicular to the surface, that is to say crossing the flattened 

 cells by their shortest diameter. The inner mass consists of cells 

 which are still more or less oblong, or which become so in process of 

 growth; and these again divide, parallel to their short axes, into 

 squarish cells, which as usual, by the mutual tension of their walls, 

 become hexagonal as seen in a plane section. There is a clear dis- 

 tinction, then, in form as well as in position, between the outer 

 covering-cells and those which he within this envelope; the latter 

 are reduced to a condition which fulfils the mechanical function of a 

 protective coat, while the former undergo less modification, and 

 become the actively Uving, reproductive elements. 



Fig. 288. Development of sporangium of Osmuuda. After Bower. 



In Fig. 288 is shewn the development of the sporangium of a 

 fern (Osmunda). We may trace here the common phenomenon of 

 a series of oblique partitions, built alternately on one another, and 

 cutting off a conspicuous triangular apical cell. Over the whole 

 system an epidermal layer , is formed, in the manner we have 

 described; and in this case it covers the apical cell also, owing to 

 the fact that it was of such dimensions that, at one stage of growth, 

 a pericKnal partition wall, cutting off its outer end, was indicated 

 as of less area than an anticlinal one. This periclinal wall cuts 

 down the apical cell to the proportions, very nearly, of an equi- 

 lateral triangle, but the solid form of the cell is obviously that of 

 a tetrahedron with curved faces ; and accordingly, the least possible 

 partitions by which further subdivision can be effected will run 

 successively parallel to its four sides (or its three sides when we 



