THE STEM AND THE LEAF 65 
commonly found in the bark of dicotyledons and are often the 
main factors in strengthening young stems. Collenchyma cells 
(A) are like the thin-walled cells of the pith, but are reénforced 
at the angles, just as some packing boxes have strips of board 
nailed fast on the inside of the box at the junctions of the sides. 
Bast fibers (B) are extremely slender tubes with closed and 
pointed ends, much like a piece of thermometer tubing drawn 
to a point in a gas flame and 
thus closed. Collenchyma gives 
moderate stiffness to the parts in 
which it occurs and is highly 
elastic, so that it does not hinder 
the growth of the stem which it 
incloses. Bast fibers are flexible 
but very tough, and therefore 
enable the parts of the root, 
stem, = leaf in which they occur Fie. 45. One quarter of a cross 
to resist being pulled apart. In “section of a stick of oak wood 
many stems, particularly those A, Seder sates, Heute tvond 
which are more than a year old bark to pith; 7, annual rings ; 0, 
(fig. 45), a great part of the total boundaries between rings, porous 
. from presence of many ducts; i, inte- 
strength is due to the presence of rior fibrous layers of dead bark; pi, 
several nds of fibers, of which Mut phe of dade gig 
the wood is largely made up. to bark beneath. Reduced 
Which stem is more like a wire 
cable in its structure, that of Dutchman’s-pipe (fig. 42, 4) 
or that of the sunflower (fig. 42, B) ? 
62. Stiffness of stems. It is a familiar fact that a metal 
tube is stiffer than a solid rod of the same kind of metal and 
the same weight per foot of length. So in many plants, just as 
in the long bones of animals, the stems are at once stiff and 
light, because the material is arranged in the form of a tube, as 
in the bamboo, the straw of the small grains, and such flower 
stalks as that of the dandelion. In other cases, as in the corn- 
stalk and in the stems of elder, the harder parts of the stem 
constitute a tube inside of which is much soft, light pith. 
