322 



THE FORMS OF TISSUES 



[CH. 



have, in addition to the forces which tend to produce the regular 

 hexagonal symmetry, some other asymmetrical component arising 

 from growth or traction, then our regular hexagons will be dis- 

 torted in various simple ways. This condition is illustrated in 

 the accompanying diagram of the epidermis of Girardia ; it also 

 accounts for the more or less pointed or fusiform cells, each still 

 in contact (as a rule) with six others, which form the epithelial 

 lining of the blood-vessels : and other similar, or analogous, 

 instances are very common. 



In a soap-froth imprisoned between two glass plates, we have 

 a symmetrical system of cells, which appear in optical section (as 



Fig. 125. Soap-froth under pressure. (After Rhumbler.) 



in Fig. 125, B) as regular hexagons ; but if we press the plates a 

 little closer together, the hexagons become deformed or flattened 

 (Fig. 125, A). In this case, however, if 

 we cease to apply further pressure, the 

 tension of the films throughout the 

 system soon adjusts itself again, and in a 

 short time the system has regained the 

 former symmetry of Fig. 125, B. 



In the growth of an ordinary dicoty- 

 ledonous leaf, we once more see reflected in 

 the form of its epidermal cells the tractions, 

 irregular but on the whole longitudinal, 

 which growth has superposed on the ten- 

 sions of the partition-walls (Fig. 126). In 

 the narrow elongated leaf of a Monocoty- 

 ledon, such as a hyacinth, the elongated, apparently quadrangular 



Fig. 126. From leaf of 

 Elodea canadensis. (After 

 Berthold.) 



