374 THE FORMS OF TISSUES [ch. 



bring about, increasing in complexity with each succeeding stage, 

 we can see, even at this advanced and complicated stage, a very 

 considerable resemblance between the actual picture (Fig. 144) 

 and the diagram which we have here constructed in obedience to 

 a few simple rules. 



In like manner, in the annexed figures, representing sections 

 through a young embryo of a Moss, we have very little difficulty 

 in discerning the successive stages that must have intervened 

 between the two stages shewn : so as to lead from the just divided 

 quadrants (one of which, by the way, has not yet divided in our 

 figure (a)) to the stage (b) in which a well-marked epidermal 

 layer surrounds an at first sight irregular agglomeration of 

 "fundamental" tissue. 



a b 



Fig. 157. Sections of embryo of a moss. (After Kienitz-Gerloff.) 



In the last paragraph but one, I have spoken of the difficulty 

 of so arranging the meeting-places of a number of cells that at 

 each junction only three cell- walls shall meet in a line, and all 

 three shall meet it at equal angles of 120°. As a matter of fact, the 

 problem is soluble in a number of ways ; that is to say, when we 

 have a number of cells, say eight as in the case considered, enclosed 

 in a common boundary, there are various ways in which their 

 walls can be made to meet internally, three by three, at equal 

 angles; and these differences will entail differences also in the 

 curvature of the walls, and consequently in the shape of the cells. 

 The question is somewhat complex; it has been dealt with by 

 Plateau, and treated mathematically by M. Van Kees*. 



If within our boundary we have three cells all meeting 



* Cit. Plateau, Statiquedes Liquides, i, p. 358. 



