524 THE FORMS OF TISSUES [ch. 



main tension; secondaries will tend to be more or less orthogonal 

 to the primary cracks; two secondaries on opposite sides of a 

 primary will tend to be near togetiier, though not opposite; and 

 spiral cracks are often to be seen, the remote effect of plasticity 

 under the potter's wheel. The net result is that certain primary 

 cracks appear, related (more or less clearly) to the circular or spiral 

 shaping of the clay upon the wheel; and each primary crack so 

 reheves the tension in one direction that the secondaries tend to 

 follow in a direction at right angles to the first. While co-equal 

 angles of 120° were likely to occur in certain symmetrical cases, 

 leading to a simultaneous pattern of hexagons, in other cases suc- 



Fig. 203. Colour-patterns of kidney-beans with diagrammatic contour-lines 

 added; a, b, Japanese "quail- beans"; c, scarlet-runner. After M. Hirata. 



cessive partitions or cracks tend to be at right angles to one another ; 

 and Sachs's Law becomes truer of the porcelain than of the plant*. 

 In this latter case, and doubtless in many more, we are deahng 

 not 'with a random pattern, but with one based on systematic and 

 predetermined lines. The apparently confused or random pattern 

 of a kidney-bean comes under the same class of configurations, 

 inasmuch as it also is based on an underlying polarity, whose 

 centre of symmetry is in the stalk or "hilus." For simphcity's 

 sake, imagine the bean round hke a pea, and its surface mapped 

 out, orthogonally, by two sets of boundary-hnes, radial and con- 

 centric. Then suppose an asymmetry of growth to be introduced 

 so that the round pea grows into the elhpsoid of a bean; and 

 suppose that the whole system of boundary-lines is subject to the 

 same conformal transformation — which elliptic functions might help 



* See H. Hukueima, Cracks upon the glazed surface of ceramic wares, Sci. 

 Papers, Inst, of Chem. and Phys. Research, Tokyo, xxvii, pp. 235-243. 1935. 



