VII] OF EPIDERMAL TISSUES 315 



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 perceive that the fundamental character 

 of an epidermis simply is that it lies on the outside, and that its 

 main physical characteristics follow, as a matter of course, from 

 the position which it occupies and from the various consequences 

 which that situation entails. We have however by no means 

 exhausted the subject in this short account; for the botanist is 

 accustomed to draw a sharp distinction between a true epidermis 

 and what is called epidermal tissue. The latter, which is found in 

 such a sea-weed as Laminaria and in very many other cryptogamic 

 plants, consists, as in the hypothetical case we have described, 

 of a more or less simple and direct modification of the general or 

 fundamental tissue. But a " true epidermis," such as we have it 

 in the higher plants, is something with a long morphological history, 

 something which has been laid down or differentiated in an early 

 stage of the plant's growth, and which afterwards retains its 

 separate and independent character. We shall see presently that 

 a physical reason is again at hand to account, under certain 

 circumstances, for the early partitioning off, from a mass of 

 embryonic tissue, of an outer layer of cells which from their first 

 appearance are marked off from the rest by their rectangular and 

 flattened form. 



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

 a plane of symmetry, and further, we have only considered the 

 appearance which they present as projected on that plane: in 

 simpler words, we have been considering their appearance in 

 surface or in sectional view. But we have further to consider 

 them as solids, whether they be still grouped in relation to a single 

 plane (like the four cells in Fig. 116) or heaped upon one another, 

 as for instance in a tetrahedral form like four cannon-balls ; and in 

 either case we have to pass from the problems of plane to those of 

 solid geometry. In short, the further development of our theme 

 must lead us along two paths of enquiry, which continually 

 intercross, namely (1) the study of more complex cases of partition 

 and of contact in a plane, and (2) the whole question of the surfaces 



