88 LIFE IN NATUKE. 



and present a hexagonal appearance, as shown in 

 Fig. 15. No one doubts that this change in the form 



Fig. 15. 



Germinal membrane, at a later period ; the cells flattened by pressure. 



of the cells is due to the pressure arising from their 

 increase under limit Can we doubt, then, that the 

 rising up of the dorsal plates is due to the same 

 cause ? in fact, that it is just such a rising up 

 as we see in the plumule of the pea? If we 

 spread a handkerchief on a table, place the hands 

 flat upon it a little way apart, and gradually bring 

 them nearer to each other, we produce similar 

 ridges. 



The frond of a common fern again illustrates the 

 process. Every one has noticed how it is curled, 



