406 THE FORMS OF TISSUES [ch. 



definitely 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 distinction, then, in form as well as in 

 position, between the outer covering-cells and those which lie 

 within this envelope ; the latter are reduced to a condition which 

 merely fulfils the mechanical function of a protective coat, while 

 the former undergo less modification, and give rise to the actively 

 living, reproductive elements. 



In Fig. 190 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. 



Fig. 190. Development of sporangium of Osmunda. (After Bower.) 



and cutting off a conspicuous triangular apical cell. Over the 

 whole system an epidermal layer has been 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 perichnal partition wall, cutting off its outer end, 

 was indicated as of less area than an antichnal one. This periclinal 

 wall cuts down the apical cell to the proportions, very nearly, 

 of an equilateral triangle, but the sohd 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 confine ourselves to the appearances as seen in 



