THE STEM 153 



have some system of canals, or some other kind of 

 passage for the conduction of saps. In order to 

 understand how the stem proceeds to serve these 

 purposes, we must study its structure and, first of 

 all, the structure of the actual cells, the bricks, as we 

 called them in our first lecture, out of which the plant 

 is built up. 



If we make a very thin transverse section of wood, 

 we at once notice that it is perforated with very minute 

 holes. These are the cavities of cells. We have already 

 studied the inside of the cell, and its chemical contents. 

 In the present instance, we are mainly interested in it as 

 building material from the mechanical point of view ; 

 and in this connection the main part is played by its 

 solid skeleton, its wall, upon which the entire form of 

 the plant depends. 



When isolated the cell is for the most part spherical 

 in form ; when connected with other cells, as is the case 

 in tissues, this spherical form passes into the polygonal 

 (as is represented on fig. 42, i) . A polygonal form shows 

 that throughout its life-time the cell has uniformly 

 developed in all directions, whereas if it develops mostly 

 in two directions, i.e. along two axes, a flat tabular form 

 will result. Such tabular cells are formed mostly at 

 the surface of organs ; they constitute the epidermis 

 of the plant (fig. 42, 2). Lastly, the cell may develop 

 almost exclusively in one direction, along one of its 

 axes only. Then instead of a polygonal or a flat, 

 tabular cell, a very narrow and elongated fibre appears, 

 (as is represented on fig. 42, 5 and 6). Such long fibres 

 form most of the wood of trees ; but even they are not 

 the very longest, and the fibre-like cells of flax, for 

 instance, which are used in spinning, are sometimes a 

 thousand times longer than their own diameter, so that 

 we could only represent them here on their true scale 

 by a single line. Cells not only vary in general 

 outline and external appearance, but the very wall of 



