Notes on Bark Structure 



155 



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

 In older age, the layers of bast libers 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, 

 arc 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. 



Juniper-us, 3 species. 



J. communis L. 



J. rirginiana L. 

 Thuja, 3 species. 



T. occidentalis L. 



T. gigantae Nutt. 

 Cupressus, 1 species. 



None American. 

 Sequoia, 1 species. 



8. gigantea Endl. 

 Larix, 2 species. 



None American. 



Bet ula, 2 species. 



None American. 

 AIn its, 2 species. 



.1. inca-na Willd. 

 Ox //-//a, 1 species. 



0. virginica L. 

 , 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. 

 Plat anus, 1 species. 



None American. 

 Liquidambar, 1 species 



None American. 

 Populus, 4 species. 



P. tremula L. 



