THE GENERAL SHAPES OF PLANTS. 121 



ness will be maintained between the two flat surfaces and 

 between the two edges. For, on the average, the relations 

 of the two flat surfaces to the sides of the openings through 

 which the diatom passes, will be alike ; and so, too, on the 

 average, will be the relations of the two edges. In 



desmids of the type exemplified by the second individual in 

 Fig. 2, a kindred equalization of dimensions is otherwise in- 

 sured. There is nothing to keep one of the two surfaces 

 uppermost rather than the other ; and hence, in the long 

 succession of individuals, the two surfaces are sure to. ba 

 similarly exposed to light and agencies in general. When 

 to this is added the fact that spontaneous fission occurs 

 transversely in a constant way, it becomes manifest that the 

 two ends, while they are maintained in conditions like one 

 another, are maintained in conditions unlike those of the two 

 edges. Here then, as before, triple bilateral symmetry in 

 form, coexists with a triple bilateral symmetry in the 

 average distribution of actions. 



Still confining our attention to aggregates of the first 

 order, let us next note what results when the two ends are 

 permanently subject to different conditions. The fixed 

 unicellular plants, of which examples are given in Figs. 4, 5, 

 and 6, beverally illustrate the contrast in shape that arises 



between the part that is applied to the supporting surface 

 and the part that extends into the surrounding medium. 

 These two parts which are the most unlike in their relations 

 to incident forces, are the most unlike in their forms. Ob- 



