Notes on Bark Structure 



147 



pith rays widen only very little, the growth in thickness must 

 occur through division of the parenchyma of the entire bark. 



Inner bark. In the secondary bark there is no character- 

 istic common to all the genera of this group. Betula, Alnus, 

 I'fdtfiniix, and Fagus are characterized by the absence of bast 

 fibers. Sclerotic bast parenchyma and bast fibers are both 

 found in Ostrya, Carpinus, Corylus, Quercus, Fagus, Castanea, 

 (V7//X, M.-n-us, Liquidambar and a few species of Populux, 

 while bast fibers only and no sclerehchyma are found in Ulmus, 

 Morns, and in some species of Salix and Populus. Several 

 genera are characterized by bast fibers which differ from the 

 usual typical appearance on account of their greater length, 

 unusual fineness, elasticity and cross sectional form; among 

 these are Ulmus, Celtis, and Morus; while in Liquidambar 

 they look somewhat like stone cells. 



The soft bast in all of these genera is composed of sieve 

 tubes, parenchyma, and crystal cells. To these elements are 

 added mucilage cells in Ulmus and latex tubes in Morus. 

 The sclerotic parenchyma cells have a characteristic appear- 

 ance in some of the genera; in Quercus they are very much 

 thickened and have very many fine pores ; while in Platanus 

 they are only slightly thickened and have broad pores. 



The variation in form of crystals and the manner of their 

 distribution throughout the inner bark allows the following 

 characteristics to be listed: 

 A. Single crystals alone or at least a crystal aggregate. 



1 



Between the sclerenchyma cells; bast fibers wanting. 



Platanus 



In sclerotic companion cells accompanying the bast 

 fiber bundles. 



Liquidambar 



Independent of details of the fiber bundles. 

 (a) Exclusively single crystals. 



B. 



(b) Sometimes a'so crystal aggregates. 



ystal aggregates always 



Ulmus 

 Celtis 



Morus 

 present 



Single crystals and 



together. 



1. Single crystals cloti:e the tangential areas of the bast 

 fiber bundles. 



