ECONOMIC WOODS OF THE UNITED STATES 35 



nature of the pits, whether simple or distinctly bordered, in the 

 walls of the wood fibres, and the character of pitting where vessels 

 are in contact with wood parenchyma or the rays, are often 

 helpful in classification. Scalariform bordered pits in the walls 

 of the vessels of Magnolia (Plate VI, Fig. 3) serve to distinguish 

 this genus from Liriodendron (Plate VI, Fig. 4), in which they 

 are absent or very sparingly developed. 



References 

 DeBary, A.: Comparative Anatomy, pp. 158-164. 

 Gregory, E. L.: The Pores [Pits] of the Libriform Tissue, Bui. Torrey Bot. 



Club, N. Y., Vol. XIII, 1886, pp. 197-204; 233-244. 

 Penhallow, D. P.: North American Gymnosperms, pp. 59-77. 

 Solereder, H. : Anatomy of the Dicotyledons, Vol. II, pp. 1139-41. 

 Gerry, Eloise: The Distribution of the "Bars of Sanio" in the Coniferales, 



Annals of Botany, Vol. XXIV, No. 93, January 1910, pp. 119-123. 

 Kreuz, J. : Die gehoften Tiipfel des Xylems der Laub- und Nadelholzer, 



Sitzb. d. Akad. Wiss., Wien, Vol. LXXVI, Part. 1, 1878, pp. 353-384. 

 Russow, E. : Zur Kenntnis des Holzes, insonderheit des Coniferenholzes, 



Bot. Centralblatt, Vol. XIII, Nos. 1-5, 1883. 



It is not uncommon to find the vessels of many Dicotyledons 

 (Plate III, Figs. 3, 4) and the resin ducts of certain Gymnosperms 

 more or less completely filled with pith-like cells called tyloses. 

 Usually the walls of the tyloses are very thin, but exceptions occur 

 (e.g., Robinia and Toxylon) where they may be considerably 

 thickened, sometimes becoming sclerotic. Tyloses in large vessels 

 are plainly visible to the unaided eye, their high lustre giving 

 them the appearance of froth. 



Tyloses are cells which have developed from protrusions of the 

 wood or ray parenchyma into the lumen of a vessel or the canal 

 of a duct or an intercellular space. Their formation is apparently 

 due to differences in pressure within the parenchyma cells and the 

 vessels or ducts they adjoin. After vessels lose their sap they are 

 no longer turgid, in fact the air within them becomes rarefied. In 

 consequence of this reduction of pressure the neighboring paren- 

 chyma cells rupture or disorganize the limiting membranes of the 

 pits, thereby rendering the lumen of the vessel available for their 

 further extension and development. This explains why tyloses 

 do not occur in vessels which are in a state of activity, but as a 



