34 THE VEGETABLE WORLD. 
essential parts, namely, pith at the centre, surrounded by woody 
fibre, and exterior bark. Let us examine more closely each of 
these parts in an indigenous tree. 
The pith forms a sort of column in the centre of the woody axis, 
as in Fig. 39, which represents an horizontal section of the trunk 
of the maple. In very 
thick and old stems, 
the diameter of the pith 
appears very small, 
and for a long time it 
“® was even supposed that 
in the trunk of very 
" 
But itis not so. It is 
j asserted that accord- 
¥_.. ing to exact measure- 
ment and observation 
the diameter retains 
é \ perceptibly and inva- 
= e riably the same pro- 
Fig. 39.—Horizontal section of a Maple-tree. portion from the time 
when the young ligneous axis has begun to solidify, up to the 
period of its maturity. The pith is formed by a combination 
of cellules, or cells, to use the scientific term. 
Cells are simple, primitive organs, which are present in every 
vegetable structure. It is a sort of sac or cavity, surrounded by 
walls of transparent membrane ; a vegetable cell, in short, is well 
represented by a soap bubble. A cellular mass without intercellu- 
lar spaces may be compared to an aggregation of soap bubbles, 
pressed against each other. The cavity is completely closed ; 
sometimes it is empty, sometimes it is filled with vegetable matter. 
Fig. 40 represents the transverse section of a cluster of young 
vegetable cells; they are, as we see, nearly circular in form. When 
they have become larger, they mutually compress each other, 
so that their form, at first nearly circular, becomes polyhedral, as 
represented in Fig 41. 
The pith of young trees, as represented in Fig. 40, is, as we have 
