8o STRENGTHENING DEVICES 



girder, masses of stereome being found on the outside of the 

 phloem and on the inside of the xylem (Fig. 46, D). The resis- 

 tance of these tissues to cutting and breaking affords abundant 

 evidence of their hardness and rigidity. It may be surprising 

 to note that the sustaining strength of the stereome fibers is 

 equal to that of the best wrought iron or hammered steel while 

 their ductility is ten to fifteen times that of iron. The superior 

 quality of these tissues and the perfection of their arrangement 

 result in structures that can not be approxiinated in any of our 

 buildings. The height of the tallest chimneys scarcely exceeds 

 the diameter of their bases more than fifteen times but many 

 stems of rushes and grasses exceed the diameter of their bases 

 from 200 to 500 times. It would be impossible with any metal 

 to construct a column of the same length and weight and having 

 the same rigidity, elasticity, and resisting power as these stemg. 

 We gain an idea of the toughness and durability of these fibers 

 when we consider that linen, rope, matting, etc., are manufac- 

 tured from them. In stems that increase greatly in diameter, 

 as our trees, the mechanical tissues are confined to the wood or 

 xylem. These are to be sure nearly solid columns, but even 

 here there is sufficient strengthening tissue in the xylem at the 

 periphery of the stem to support the trunk, as is often attested 

 by the sturdy character of trees that have become hollow. 



37. The Secondary Growth of the Stem. — -The arrangemetn 

 of the tissues as outlined above, termed the primary growth, 

 remains practically unchanged in some annual plants but in such 

 forms as live on from year to year or that increase materially 

 in diameter there results extensive alterations in the structure 

 of the stems owing to the formation of new tissues, especially 

 in the region of the cortex and in the vascular bundles. The 

 epidermis furnishes sufficient protection to such stems as do not 

 increase materially in size, such as the majority of our annual 

 plants; but in long lived stems, as shrubs and trees, where growth 

 goes on from year to year the epidermis is not able to keep pace 

 with the growth. 



38. Cork Tissue. — -To meet this condition a new tissue, the 

 cork, is developed from certain cells called the cork cambium, 



