Notes on Bark Structure 151 



in the pith rays; sieve tubes wide with very large and net- 

 like sieve fields. 



Fraxinus 

 Catalpa. 



Outer bark. The periderm is always developed early in 

 the outermost layer of cells of the primary bark. The cells of 

 the cork layer are large, slightly flattened and thin-walled. 

 ('(ilalfHt has scaly bark. 



Middle bark. The primary bark is of typical collenchyma, 

 and contains no crystals. It never becomes sclerotic. 



Inner bark. The secondary bark contains bast fibers in 

 concentric tangential bands, but no stone cells. The soft bast 

 consists mainly of parenchyma and contains an abundance of 

 calcium oxalate in the form of either crystal sand, raphides, 

 or small prisms, without reference to the bast fibers. The pith 

 rays are never more than four rows wide, are never sclerotic 

 and are filled with the same form of crystals as those found 

 in the bast parenchyma. 



The most striking characteristic is the absence of stone 

 cell formation in the middle #nd inner bark. 

 Summary : 



Bast fibers never accompanied by crystals, no stone 

 cells in the secondary bark; pith rays always thin walled. 

 Bast fibers in tangential bands broken only by pith rays; 

 the latter of several rows of cells containing an abundance 

 of raphides and crystal sand as does also the bast paren- 

 chyma. The layers of soft bast much broader than those 

 of the bast fibers. 



Catalpa 



Magnoliaceae. 



Outer bark. The periderm develops early. The phello- 

 gen originates in the layer of cells immediately adjoining the 

 epidermis. The superficial periderm consists of a few rows of 

 thin-walled cells. The inner periderm is characterized by a 

 layer-like thickening of the cork. 



Middle bark. A closed although rather weak hypoderma 

 )f collenchyma is formed. Short secretion cells are especially 

 characteristic. Sclerotic idioblasts are formed in the middle 

 irk of Magnolia. Calcium oxalate is present in Liriodendron 

 the form of fine sand. 



