372 THE FORMS OF TISSUES [ch. 



be an arc of a circle of which we can determine the centre by the 

 method used on p. 307 ; and, furthermore, the narrower cells, 

 that is to say the quadrilaterals, will have this outer border 

 somewhat more curved than their broader neighbours. We arrive, 

 then, at the condition shewn in Fig. 153, b. Within the cell, 

 also, wherever wall meets wall, the angle of contact must tend, 

 in every case, to be an angle of 120° ; and in no case may more 

 than three films (as seen in section) meet in a point (c) ; and 

 this condition, of the partitions meeting three by three, and at 

 co-equal angles, will obviously involve the curvature of some, if 

 not all, of the partitions (d) which in our preliminary investigation 

 we treated as plane. To solve this problem in a general way is 

 no easy matter; but it is a problem which Nature solves in 

 every case where, as in the case we are considering, eight bubbles, 

 or eight cells, meet together in a (plane or curved) surface. An 

 approximate solution has been given in Fig. 153, d ; and it will now 

 at once be recognised that this figure has vastly more resemblance 

 to an aggregate of living cells than had the diagram of Fig. 153, a 

 with which we began. 



Just as we have constructed in this case a series of purely 

 diagrammatic or schematic figures, so it will be as a rule possible^ 

 to diagrammatise, with but little alteration, the 

 complicated appearances presented by any ordinary 

 aggregate of cells. The accompanying little figure 

 (Fig. 154), of a germinating spore of a Liverwort 

 (Riccia), after a drawing of Professor Campbell's, 

 ^' "^ ' scarcely needs further explanation : for it is well-nigh a 

 typical diagram of the method of space-partitioning which we are 

 now considering. Let us look again at our figures (on p. 359) of the 

 disc of Erythrotrichia, from Berthold's Monograph of the Bangiaceae 

 and redraw the earlier stages in diagrammatic fashion. In the 

 following series of diagrams the new partitions, or those just about 

 to form, are in each case outhned ; and in the next succeeding 

 stage they are shewn after setthng down into position, and after 

 exercising their respective tractions on the walls previously laid 

 down. It is clear, I think, that these four diagrammatic figures 

 represent all that is shewn in the first five stages drawn by 

 Berthold from the plant itself; but the correspondence cannot 



