222 The Living Plant 



position, but a definite arrangement in the stem, where they 

 lie in a ring, as our pictures illustrate (figures 73, 139 B). The 

 tissue in which they are embedded consists mostly of thin-walled 

 cells, of rounded or polyhedral shapes. The part thereof lying 

 inside of the ring of bundles makes up the pith, which is com- 

 monly utilized for storage; that between the bundles constitutes 

 the beginnings of structures later to be considered as the medul- 

 lary rays; while the tissue outside of the bundles forms the cortex, 

 which contains some chlorophyll, and aids in the photosynthetic 

 work. This cortex, by the way, is continuous and morphologically 

 identical with the green tissue of the leaf ; and one (3an form a very 

 useful and reasonably accurate conception of the anatomical 

 relations of stem and leaf by imagining that one of the fibro- 

 vascular bundles of the stem is snipped out from among its 

 neighbors, and, with its adherent cortex, bent outward at right 

 angles to the stem and then flattened and fringed out to a network 

 which the green tissue surrounds and fills in. But as to our 

 stem, outside of the tissues aforementioned comes the single layer 

 of epidermis, physiologically the plant's skin, with its distinctive 

 flat chlorophylless cells pierced here and there by the stomata. 

 Finally, sometimes in connection with the sieve-tubes, some- 

 times as a ring or as scattered islands in the cortex, or just under 

 the epidermis, occur masses of very thick-walled cells, showing 

 long and pointed when seen lengthwise, which are the important 

 fibers that give strength to the stem. Howsoever these fibers are 

 distributed, there is always one constant feature about their posi- 

 tions, that they tend to keep close towards the outside of the 

 stem. And the reason therefor is sufficiently plain, — ^it is a 

 fundamental principle of mechanics that any given amount of 

 strengthening material exerts its greatest supporting effect against 

 lateral strains if disposed in the form of a hollow cylinder or tube, 

 which is the reason why columns used in building construction 

 are hollow, not solid, why a bicycle frame is constructed of tubes, 

 not of rods, and why a great tree can stand as a mere shell of 



