36 ECONOMIC WOODS OF THE UNITED STATES 



general rule arise in the inner region of the sapwood, i.e., in the 

 wood where the vessels are losing their power of conduction. Once 

 inside the vessel, the intruding cells rapidly divide and grow until 

 the space is filled or their food-supply is exhausted, and thus form 

 a parenchymatous tissue in which carbohydrates may be stored. 



The effect of the formation of tyloses is to block up the vessels 

 and render the heartwood impervious, or nearly so, to the entrance 

 of fluids. Tyloses are especially abundant in the vessels of white 

 oaks (Frontispiece), thus adding to the technical value of the 

 wood for cooperage. This feature is also of some value in sepa- 

 rating the white from the black oaks, since in the latter group 

 tyloses are rather scarce or wanting (Plate II, Fig. 6). In Quercus 

 marilandica, however, tyloses are abundant. 



Tyloses also occur occasionally in the tracheids of the wood of 

 Gymnosperms, particularly in the wood of the roots. Tyloses 

 in resin ducts are characteristic of Pinus and (in less degree) Picea, 

 but are sparingly developed or absent in Larix and Pseudotsuga. 



References 

 KIRSCH, SIMON: On the Development and Function of Certain Structures in 



the Stipe and Rhizome of Pteris aquilina and Other Pteridophytes, 



Trans. Royal Soc. of Canada, 3d Series, Vol. I, sec. iv, Ottawa, 1908, 



pp. 403-8. 



DfiB ART, A.: Comparative Anatomy, p. 170. 

 SACHS, JULIUS: Lectures on the Physiology of Plants (trans, by H. Marshal 



Ward), p. 581. 

 CHRYSLER, M. A.: Tyloses in Tracheids of Conifers, New Phytologist, No. 7, 



1908, pp. 198-204. 

 GOLDEN, K. E.: Tyloses in Brosimum aubletii, Proc. Ind. Acad. Sci., 1904, 



pp. 227-232. 

 VON ALTEN, H. : Kritische Bemerkungen und neue Ansichten iiber die Thyllen, 



Bot. Zeitung, Vol. LXVII, 1909, pp. 1-23. 

 RAATZ, WILHELM : Ueber Thyllenbildungen in den Tracheiden der Conif eren- 



holzer, Ber. d. deutschen Bot. GeseUschaft, Vol. X, 1892, pp. 183-192. 



PITH FLECKS OR MEDULLARY SPOTS 



Pith flecks or medullary spots are small, brown or grayish, 

 half-moon-shaped patches appearing so commonly on the cross 

 sections of many diffuse-porous woods, especially of the four 

 families Salicacece, Betulaceoe, Rosacece, and Aceracece. On longi- 



