494 



THE FORMS OF TISSUES 



[CH. 



a rule, the same kind of contact (that is to say, contact with air) 

 both within and without the bubbles; while in our hving cell, the 

 outer wall of the epidermal cell is exposed to air on the one side, 

 but is in contact with the protoplasm of the cell on the other : and 

 this involves a difference of tensions, so that the outer walls and 

 their adjacent partitions need no longer meet at precisely equal 

 angles of 120°. Moreover a chemical change, due perhaps to 

 oxidation or possibly also to adsorption, is very apt to affect the 

 external wall and lead to the formation of a "cuticle"; and this 

 process, as we have seen, is tantamount to a large increase of tension 

 In t^at outer wall, and will cause the adjacent partitions to impinge 

 upon it at angles more and more nearly approximating to 90°: the 

 bubble-hke, or spherical, surfaces of the individual cells being more 



Fig. 177. A froth, with its outer and inner cells or vesicles, 



and more flattened in consequence. Lastly, the chemical changes 

 which affect the outer walls of the superficial cells may extend in 

 greater or less degree to their inner walls also : with the result that 

 these cells will tend to become more or less rectangular throughout, 

 and will cease to dovetail into the interstices of the next subjacent 

 layer. These then are the general characters which we recognise 

 in an epidermis ; and we now perceive that its fundamental character 

 simply is that it lies outside, and that its physical characteristics 

 follow, as a matter of course, from the position which it occupies 

 and from the various consequences which that situation entails. 



In the young shoot or growing point of a flowering plant botanists 

 (following Hanstein) find three cell-layers, and call them dermatogen, 

 periblern and plerome. The first is an epidermis, such as we have 

 just described. Its cells grow long as the shoot grows long; new 

 partitions cross the lengthening cell and tend to lie at right angles 

 to its hardening walls; and this epidermis, once formed, remains 

 a single superficial layer. The next few layers, the so-called peri- 



