TOPOGRAPHY OF SKELETON 



85 



In roots the xylem is placed closer to the center than in stems, 

 and by crowding in very soon obliterates the pith and assumes 

 the form of a solid rod. This difference of the wood skeleton 

 in the stem and the root is related 

 to the difference in the direction of 

 the stresses which they have to 

 overcome. In the stem it is the 

 weight of the crown and the alter- 

 nate stretching and compressing 

 when swaying in the wind; while in 

 roots it is the pulling force which 

 the swaying stem exerts on the roots, 

 and the compressing force with which the soil resists growth in 

 thickness of the roots, and both of these stresses the solid wood 

 cylinder of roots is well adapted to withstand. 



In most monocotyledonous stems the problem of locating 

 the skeletal tissues is simplified because no allowance needs to 

 be made for secondary increase in the vascular bundles, and 

 seldom for secondary increase in the stem as a whole, and this 

 fact is taken advantage of by encasing, and so bracing and 



Fig. 39. — Camera-lucida outline 

 of ^portion of cross section of corn- 

 stalk, showing at g bast fiber zone 

 beneath the epidermis and surround- 

 ing the outermost vascular bundles. 



g £•'«' d 



Pig. 40. — Cross section of a portion of palm stem, e, xylem; /, phloem portions of 

 vascular bundle; g, solerenohyma tissue about vascular bundle; d, fundamental or ground 

 tissue; c, larger tracheal tubes in vascular bundle. (After Engler and Prantl.) 



strengthening each vascular bundle in a sheath of sclerenchyma 

 or bast fibers developed from the ground parenchyma tissues 

 (Figs. 28 and 40). 



In grasses and similar Monocotyledons many vascular bun- 

 dles are massed close to the epidermis, each with its protecting 



