NATURE OF PLANTS 71 



hammered steel while their ductility is ten to fifteen times that 

 of iron. The superior quality of these tissues and the perfec- 

 tion of their arrangement result in structures that can not be 

 approximated 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 stems. We gain an idea of the tough- 

 ness and durability of these fibers when we consider that linen, 

 rope, matting, etc., are manufactured 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 colums, 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. 



36. The Secondary Growth of the Stem. The arrangement 

 of the tissues as outlined above, termed true 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. 



37. Cork Tissue. To meet this condition a new tissue, the 

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

 usually situated near the epidermis (Fig. 46). The cells of the 

 cork cambium divide much after the manner noted in the cam- 

 bium of the vascular bundle, but there is this difference, the outer 

 cell of the two daughter cells becomes a cork cell while the inner 

 cell remains capable of further division. Only rarely does the 



