52 The New York State College of Forestry 



but in such ringbarked species as Birch and Cherry they may per- 

 sist for many years as enlarged and horizontally elongated bodies 

 ^vhich measure an inch or more in length. 



G. Pith 



With the exception of the tree ferns, the arborescent Monoco- 

 tyledons, certain of the primitive Gymnosperms of dendroid habit, 

 and scattered dicotyledonous species, trees are characterized by 

 woody stems which possess a central pith or medulla and radial 

 symmetry, and thicken by the addition of yearly increments which 

 take the form of annual or seasonal rings as seen in cross section. 

 The pith differs from the wood not merely in position but in the 

 nature of the cells which constitute it. These are usually more or 

 less globose or isogonal, often thin-walled and rather loosely aggre- 

 gated, and reflect the light differently as seen in cross-sections of 

 the twig. The function of pith is in doubt. In twigs or small 

 branches it undoubtedly acts as a storage organ for reserve food 

 during the winter but in older organs it apparently has no such 

 function and remains as an obsolete tissue surrounded by heart 

 wood. 



The value of the pith in twig identification lies in its con- 

 sistency, shape, and color. In the majority of woody plants it 

 is homogeneous (Fig. 70) as seen in vertical section without cross- 

 i)artitions or interruptions of any kind. In other cases it may 

 have disappeared entirely leaving a hollow stem although this 

 is unusual in trees but is commonly found in shrubs such as the 

 Elders and Honeysuckles, The Black Walnut and Butternut 

 have a diaphragmed or chambered pith (Pig. 69) which consists 

 of many thin dissepiments spanning the pith cavity at brief 

 intervals. Other trees such as the Hackberry and Tupelo possess 

 a diaphragmed-stuffed pith in which the soft, homogeneous tissue 

 which fills most of the pith cavity is interrupted by plates of 

 denser tissue (stone cells). 



The outline of the pith in cross-section varies in different kinds 

 of trees, a character which may sometimes be used in their identi- 

 fication. Oaks, Chestnut, and Aspens are characterized by a .star- 

 shaj)tHl or .l-angled i)ith (Fig. 73), while the medulla is 3-angled 

 in Alder (Fig. 74), features related respectively to 2/5 and % 

 phyllotaxies. In many species such as the Elm and the Magnolia 

 the outline is circular (Fig. 71) or somewhat lopsided as in Bass- 

 wood. (Pig. 72.) The size and color of the pith, especially in 



