Notes on Bark Structure 



or even larger part of the inner bark than do the bast fibers. 

 In older age, the layers of bast fibers are formed only at inter- 

 vals of many years. The fibers are thin, smooth and long 

 pointed. The sieve tubes, with simple horizontal cross-plates, 

 are arranged in layers alternating with layers of parenchyma. 

 The former are thinner walled. The crystal cells always con- 

 tain rhomboidal single crystals. Pith rays are three to five 

 rows wide; the cells with thinner walls than the bast paren- 

 chyma and stretching radially. The cell walls where immedi- 

 ately joining sclerotic cells do not become thickened. 

 Summary : 



The secondary bark contains layers of bast fibers alter- 

 nating with larger layers of stone cells. The former are 

 surrounded by crystal cells. Pith rays <are broad. 

 Summary : 



Acer 



List of genera mentioned, the number of species of each 

 that were studied, and what American species investigated. 



CONIFERS. 



Juniperus, 3 species. 



J. communis L. 



J. virginiana L. 

 Thuja, 3 species. 



T. occidentalis L. 



T. gigantae Nutt. 

 Cupressus, 1 species. 



None American. 

 Sequoia, 1 species. 



S. gigantea Endl. 

 Larix, 2 species. 



None American. 



Betula, 2 species. 



None American. 

 Alnus, 2 species. 



A. incana "Willd. 

 Ostrya, 1 species. 



0. virginica L. 

 Carpinus, 1 species. 



None American. 



Abies, 2 species. 



A. canadensis Mill. 

 Picea, 1 species. 



None American. 

 Taxus, 1 species. 



None American. 

 Taxodium, 1 species. 



T. distichum Rich. 

 Pinus, 5 species. 



P. strobus L. 



BROADLEAF. 



Corylus, 2 species. 



None American. 

 Platanus, 1 species. 



None American. 

 Liquidambar, 1 species 



None American. 

 Populus, 4 species. 



P. tremula L. 



