ECONOMIC WOODS OF THE UNITED STATES 37 



tudinal sections of a stem the pith flecks appear as flattened strands 

 running up and down the stem, and often into the root. Examined 

 microscopically, pith flecks are seen to be made up of irregularly 

 shaped, polyhedral, parenchymatous cells with thick, dark- 

 colored walls copiously pitted with simple pits. At certain seasons 

 the cells are filled with starch grains. 



Pith flecks have a pathological origin. They are due to the 

 work of cambium miners whose tunnels are filled by the tylosal 

 development of adjacent uninjured parenchyma cells, especially 

 of the cortex. The dissolved cell fragments and larval excrement 

 are pressed into a narrow border by the rapid growth and division 

 of the "filling cells." 



This feature has frequently been used for purposes of classifi- 

 cation, principally because of the failure to understand its exact 

 nature. It has been noted in a large number of woods, but is by 

 no means constant in its occurrence. Some stems, for example, 

 contain numerous pith flecks, while other individuals of the same 

 species in the vicinity, or even from the same root stock, do not 

 show them. Furthermore, in stems with pith flecks certain growth 

 rings may be free of them, while others of the same section are 

 thickly dotted, or the lower portion of the stem may contain 

 them and the upper be entirely free. 



Taken in connection with other features, however, the presence 

 of pith flecks in abundance may serve to indicate the species. 

 For example, they are usually very numerous in Betula populifolia 

 and B. papyrifera, and infrequent in B. lenta, B. lutea, and B. nigra, 

 numerous in Acer rubrum and A. saccharinum, but usually want- 

 ing in A. saccharum. 



References 



Record, S. J.: Pith Flecks or Medullary Spots in Wood, Forestry Quarterly, 



Vol. IX, No. 3, 1911, pp. 244-252. 

 Grossenbacher, J. G.: Medullary Spots: A Contribution to the Life History 



of Cambium Miners, Tech. Bui. No. 15, N. Y. Agri. Exp. Sta., Geneva, 



N. Y., 1910. 

 Von Tubeuf, Karl F.: Die Zellgiinge der Birke und anderer Laubholzer, 



Forstlich-naturw. Zeitschrift, VI. Jahrgang, 1897, pp. 314-319. Also 



Naturw. Zeitschrift f. Forst- und Landwirtschaft, 6. Jahrgang, 1908, 



pp. 235-241. 

 Kienitz, M.: Die Entstehung der " Markflecke," Bot. Centralblatt, Vol. 



XIV, 1883, pp. 21-26; 56-61. 



