Notes on Bark Structure 149 



(b) Numerous sieve plates arranged ladder-like. 

 (x) Pith rays in one row 



Alnus 

 (y) Pith rays in several rows. 



Betula 



1>. Bust fibers present, but there are none, or only very few 

 stone cells. 



1. Extensive, closely packed bast fiber bundles in tan- 



gential rows accompanied by crystal cells; sieve 

 tubes with ladder-form plate system; pith rays of one 

 row. 

 (One species of Populux develops sclerenchyma.) 



Salix 

 Populus 



2. Small bast fiber bundles, loosely grouped and inde- 



pendent of crystal cells ; sieve tubes with simple cross- 

 plates; pith rays with more than one row. 



(a) Mucilage cells in soft bast; no stone cells. 



Ulmus 



(b) Latex tubes in the soft bast; sclerenchyma 

 present. 



Morus 

 ( '. Hast fibers and sclerenchyma both always present. 



1. Bast fiber bundles grouped exclusively in large con- 



centric tangential rows, and accompanied by crystal 

 cells. 



(a) Sclerenchyma predominating; pith rays broad, 

 may become sclerotic, any place. 



Quercus 



(b) Regularity of the layers not influenced by the 

 small amount of sclerenchyma present; pith rays 

 one row. 



Castanea 



2. Hast fibers short and knotty combined with stone 



cells into plates and accompanied by crystal cells; 

 pith rays of several rows. 



Liquidambar 



.'}. The rows of hast fi'm-r bundles frequently indistinct, 

 (a) The bast fiber bundles accompanied by crystal 

 cells; 



