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 reenforced 



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 (.#) 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 growtli of the stem which it 



incloses. Bast fibers are flexible 



but very tough, and therefore 



enable the parts of the root, 



stem, or leaf in which they occur 



to resist being pulled apart. In 



many stems, particularly those 



which are more than a year old 



(fig. 45), a great part of the total 



strength is due to the presence of 



several kinds of fibers, of which 



the wood is largely made up. 



Which stem is more like a wire 



cable in its structure, that of Dutchman's-pipe (fig. 42, A} 

 or that of the sunflower (fig. 42, H) ? 



62. Stiffness of stems. It is a familiar fact that a metal 

 tube is stiff er 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. 



FIG. 45. One quarter of a cross 

 section of a stick of oak wood 



m, medullary rays, running from 

 bark to pith ; r, annual rings ; b, 

 boundaries between rings, porous 

 from presence of many ducts; i, inte- 

 rior fibrous layers of dead bark ; pi, 

 hard plates of dead bark splitting 

 away from each other but attached 

 to bark beneath. Reduced 



