84 THE PLANT SKELETON 



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surface could not long be maintained without its breaking 

 asunder due to the new tissues formed by the cambium, or, if 

 too strong for this, without preventing increase in diameter. 

 These conditions, however, we find are happily met. Next the 

 epidermis is placed the collenchyma, in the form of a hollow 

 cylinder or in separate strands. It is not too strong to resist 

 growth in diameter; and, since it is chiefly for temporary service 

 until the bast and wood fibers have been laid down, it can without 

 detriment be broken apart or tangentially stretched as growth 

 in diameter proceeds. 



The bast fiber tissue is, as a rule, near to the surface, but in 

 isolated strands, in order that increase in diameter and the 

 flow of water and sap radially to and fro may not be too much 

 interfered with. The strands of bast fibers that stand in front 

 of and against the phloem serve the double function of skeleton 

 for the stem as a whole and for the thin-walled phloem tissues 

 in particular which they sustain as the bones do the weaker 

 tissues of the vertebrate body. In Fig. 17 different plans of col- 

 lenchyma and bast fiber topography are shown. 



The secondary xylem in many herbaceous and all woody 

 Dicotyledons and Gymnosperms furnishes a skeleton that is 

 especially adapted to secondary increase in thickness, since it 

 is located inside the cambium ring and can be increased indefi- 

 nitely without hindering the growth of other tissues or being 

 itself subjected to stresses that tend to tear it asunder. It can 

 therefore form the continuous cylinder that is so highly desir- 

 able in the plant skeleton. 



In many herbaceous plants the xylem cylinder surrounds a 

 relatively large pith, and so conforms in the measure that other 

 functions will permit to the accepted principles of mechanical 

 construction where economy of materials is desired; and in 

 woody perennials the xylem that is first formed is placed outside a 

 pith of greater or less dimensions, but secondary increase in thick- 

 ness after a few years so far surpasses the original diameter of the 

 stem that whether the xylem cylinder remains hollow or crowds 

 in and crushes out the pith becomes a matter of no significance. 



