VII] OF EPIDERMAL TISSUES 495 



blem, are compressed and flattened between the epidermis with its 

 tense cuticle and the growing mass within ; and mider this restraint 

 the cell-layers of the periblem also continue to divide in their own 

 plane or planes. But the cells of the inner mass or plerome, lying 

 in a more homogeneous field, tend to form " space-fiUing " poly- 

 hedra, twelve- or perhaps fourteen-sided according to the freedom 

 which they enjoy. In a well-known passage Sachs declares that 

 the behaviour of the cells in the growing point is determined not 

 by any specific characters or properties of their own, but by their 

 position and the forces to which they are subject in the system of 

 which they are a part*. This was a prescient utterance, and is 

 abundantly confirmed f. 



We have hitherto considered our cells, or our bubbles, as lying 

 in a plane of symmetry, and have only considered their appearance 

 as projected on that plane; but we must also begin to consider 

 them as sohds, whether they lie in a plane (hke the four cells in 

 Fig. 172), or are heaped on one another, like a froth of bubbles or 

 a pile of cannon-balls. We have still much to do with the study 

 of more complex partitioning in a plane, and we have the whole 

 subject to enter on of the solid geometry of bodies in "close 

 packing," or three-dimensional juxtaposition. 



The same principles which account for the development of 

 hexagonal symmetry hold true, as a matter of course, not only of 

 cells (in the biological sense), but of any bodies of uniform size and 

 originally circular outline, close-packed in a plane; and hence the 

 hexagonal pattern is of very common occurrence, under widely 

 varying circumstances. The curious reader may consult Sir Thomas 

 Browne's quaint and beautiful account, in the Garden of Cyrus, of 

 hexagonal, and also of quincuncial, symmetry in plants and animals, 

 which "doth neatly declare how nature Geometrizeth, and observeth 

 order in all things," 



We come back to very elementary geometry. The first and 

 simplest of all figures in plane geometry (wdth which for that reason 

 Euclid begins his book) is the equilateral triangle; because three 

 straight lines are the least number which enclose two-dimensional 



* Lectures on the Physiology of the Plant, Oxford, 1887, p. 460, etc. 

 t Cf. J. H. Priestley in Biol. Reviews, m, pp. 1-20, 1928; U. Tetley in Ann. Bot. 

 L, pp. 522-557, 1936; etc. 



