VII] OF EPIDERMAL CELLS 509 



partition walls (Fig. 188). In the narrow elongated leaf of a mono- 

 cotyledon, such as a hyacinth, the elongated, apparently quad- 

 rangular cells of the epidermis appear as a necessary consequence 

 of the simpler laws of growth which gave its 

 simple form to the leaf as a whole. In all 

 these cases ahke, however, the rule still 

 holds that only three partitions (in surface 

 view or plane projection) meet in a point; 

 and near their point of meeting the walls are 

 manifestly curved for a little way, so as to 

 permit the triple conjunction to take place 

 at or near the co-equal angles of 120°, after 

 the fashion described above. 



Briefly speaking, wherever we have a system j^^jg ^g^ Epidermal cells 

 of cylinders or spheres, associated together from leaf of Elodea 



with sufficient mutual interaction to bring canadensis. After 



, ^ Berthold. 



them into complete suriace contact, there, 



in section or in surface view, we tend to get a pattern of hexagons. 

 In thickened cells or fibres of bast or wood, the " sclerenchyma " 

 of vegetable histology, the hexagonal pattern is all but lost, and we 

 see in cross-section the more or less circular transverse outlines of 

 elongated and tapering cells. Looking closer we see that the 

 primitive cell-walls preserve their angular contours, and shew much 

 as usual an hexagonal pattern, with only such irregularities as 

 follow from the unequal sizes of the associated cells. But when 

 these primary walls are once laid down, the secondary deposits which 

 follow them are under different conditions; and these obey the law 

 of minimal areas in their own way, by filling up the angles of the 

 primary cell and by continuing to grow inwards in concentric and 

 more and more nearly circular rings. 



While the formation of an hexagonal pattern on the basis of ready-formed 

 and symmetrically arranged material units is a very common, and indeed the 

 general way, it does not follow that there are not others by which such a 

 pattern can be obtained. For instance, if we take a little triangular dish of 

 mercury and set it vibrating (either by help of a tuning-fork, or by simply 

 tapping on the sides) we shall have a series of little waves or ripples starting 

 inwards from each of the three faces; and the intercrossing, or interference 

 of these three sets of waves produces crests and hollows, and intermediate 

 points of no disturbance, whose loci are seen as a beautiful pattern of minute 



