8 ECONOMIC WOODS OF THE UNITED STATES 



pith is conr[mratively lai-gc and conspicuous, often deeply colored, 

 rh Magnolia, Liriodendron, I^yssa, Asimina, and Anona there is 

 often a more or less distinct septation of the continuous pith 

 ])y plates of stone cells, while in Jiiglans there is decided septation 

 but the diaphragms are not sclerotic, and the pith is not con- 

 tinuous between the disks. On account of these and other pecu- 

 liarities the pith when present in a specimen of wood is frequently 

 an aid to identification. 



References 



DkBauy, a.: Comparative Anatomy, Oxford, 1884, pp. 402-403; 533-534. 

 FoxwouTHY, J. W.: Discoid Pith in Woody Plants, Proc. Indiana Academy 



of Science for 1903, Indianapolis, 1904, pp. 191-194. 

 SoLEKKDKK, IIans: Systematic Anatomy of the Dicotyledons, Oxford, 1908, 



pp. 133-134. 

 Gris, a.: Sur la Moelle des Plantes Ligneuaes, Amer. Sci. Nat., S6r. 5, 



Tome XIV, 1872. 



BARK 



Bark is the name commonly applied to that portion of a 

 stem lying outside the cambium layer. Used in this broad sense, 

 it is customaiy to distinguish an outer (dry) portion and an inner 

 (living) portion. The structure of bark is highly complex and 

 widely variable. 



When shoots are first formed they are covered by a very 

 thin layer of tissue, the epidermis. Beneath this is the primary 

 cortex and the pericycle. The latter is commonly composed of two 

 kinds of tissues, thin-walled parenchyma and bast-fibres. The bast- 

 fiV)res may occur in isolated groups or form a continuous band 

 around the stem. When in groups they are often closely associated 

 with, but not really part of, the phloem of the vascular bundles. 

 Bust-fibres are attenuated sclerenchymatous elements, with 

 sharp ends simple or branched. Their function is to give 

 strength to the stem and to protect the delicate tissues of the 

 phloem. It is to them that many barks owe their great tough- 

 ness and pliability. 



Phloem^ which is the outer portion of a vascular bundle, is in 

 typical cases composed of sieve tubes, companion cells, and phloem 

 parenchyma. In structure sieve tubes resemble vessels, but their 

 walls are mostly delicate, non-lignified, colorless, cellulose mem- 



