900 A TEXTBOOK OF THEORETICAL BOTANY 



tyloses in wood vessels, in which case the cells have manifestly only come 

 into contact at a late stage of development. 



The last class of fibres is that called substitute fibres, which are really 

 living cells, often containing starch, but with lignified walls. They are not 

 clearly separated, except in point of length, from xylem parenchyma, and 

 indeed they are sometimes divided by transverse walls forming a short chain 

 of typical parenchyma. 



Fig. 883. — Quercusrobur. Photograph of a model showing 

 a three-dimensional view of the structure of the 

 secondary wood. On the right, longitudinal radial 

 view ; on the left, longitudinal tangential view. 



The xylem parenchyma cells arise by repeated transverse divisions of 

 cambial initials. Their walls are lignified but not heavily, and they are 

 richly supplied with simple pits on the radial and horizontal walls. They 

 are important as stores of reserve substances, either starch, in most hard- 

 wooded trees, or oil in soft-wooded trees. 



They also form important links between the cells of adjacent medullary 

 rays. Careful investigation shows that they are seldom isolated, but occur 

 in irregular tracts, which contact one or more of the medullary rays, usually 

 two, between which they form a tangential bridge. In this way they serve 

 to make an intercommunicating system of living cells in the wood, providing 

 for tangential transport, as the medullary rays themselves do for radial 

 transport. 





