160 THE INDIVIDUAL ORGANISM 



obliterated. Masses of these lifeless fibers, packed side by side, form the 

 tissue called sclerenchyma — the strongest mechanical tissue developed 

 by the plant. The strands of sclerenchyma lie longitudinally in the xylem 

 and so do not hinder the transport of water. 



The conductive tissue is made up of two principal types of cells. The 

 tracheids are spindle-shaped and relatively short. They are arranged in 

 long strands of overlapping cells, and have pitted walls that permit water 

 to pass from cell to cell. The tracheae are much larger, elongate, tubular 

 cells. They are arranged end to end in vertical alignment, with the end 

 walls of the cells perforated or dissolved away so that a row of tracheae 

 forms a continuous tube. Such many-celled tubes are called vessels. The 



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Fig. 11.7. Cross sections of ring-pored wood of red oak (left) and diffuse-pored wood of hard 

 maple (right). (Courtesy General Biological Supply House, Inc.) 



length of vessels varies; in diffuse-pored woods such as apple and maple, 

 individual vessels range from a fraction of an inch to perhaps 6 feet in 

 length, while in ring-pored woods like oak and ash some vessels run 

 unbroken the full length of the trunk and main branches. 1 Each mature 

 tracheid or trachea consists merely of the wall of a dead cell or cells, the 

 protoplasm having died and disintegrated. It is only after the death of 

 the cell that the lifeless cell walls can become functional conduits for 

 water transport. 



The phloem. Unlike xylem, the phloem is composed mainly of living 

 cells. The principal conducting elements are the sieve tubes — vertical 

 rows of large, elongate cells with perforated end walls through which 

 protoplasmic strands connect one cell with the next. It is these perforated 

 partitions that give the sieve tubes their name. Commonly the mature 



1 There are probably never any open tubes connecting the leaves with the vessels 

 of the stem. 





