34 ECONOMIC "WOODS OF THE UNITED STATES 



This membrane, which is really made up of two membranes of 

 contiguous cells which have become united in development, is 

 very thin toward the border of the pit, but usually thickened 

 near the centre. This thickened portion is called the torus (Fig. 

 11, t). The pit membrane very frequently increases in size and 

 bulges out so that the torus lies lid-like against the aperture of 

 the pit canal (Fig. 11, A'). A sieve-like structure of the pit 

 membranes has been observed in the bordered pits of the vessels 

 in certain species.* 



Between the bordered pits on the radial walls of the tracheids 

 of Gymnosperms it is very common to find folds of cellulose, which, 

 when properly stained, are quite conspicuous under the compound 

 microscope. These folds, which appear as horizontal or more or 

 less semi-circular markings, sometimes doubled, are most abundant 

 in the thin-walled tracheids of the early wood. They are without 

 diagnostic value. 



The apparent funetion of pits is to facilitate the passage of some 

 part of the cell contents from one cell to another. Bordered pits 

 are mostly associated with water transfer, and simple pits with 

 the distribution of elaborated food. 



Pits are of considerable value for systematic purposes. For 

 example, in the white pines and Pinus resinosa, the radial wall 

 of each ray-parenchyma cell shows one or two large simple pits 

 communicating with each adjacent wood tracheid, while in the 

 foxtail and nut pines and in the hard pines there are three to 

 six rather small pits so communicating (Figs. 4-7). The presence 

 of pits in the tangential walls of the tracheids of the late wood 

 in soft pines, and their absence in similar location in the pitch 

 pines, serve as an additional point of distinction between these 

 two great groups. 



While the pits in the radial walls of the tracheids of Gymno- 

 sperms are usually in a single row, they may occur in biseriate 

 or triseriate arrangement. In the larger tracheids of Tsuga they 

 are mostly biseriate. In Taxodium distichum they are characteris- 

 tically crowded, flattened, and often irregularly arranged. 



In dicotyledonous woods as a whole, pits are much smaller 

 and less regular in their distribution than in Gymnosperms. The 



* JOnsson, Bengt. : Siebahnliche Poren in den trachealen Xylemele- 

 menten der Phanerogamen, hauptsachlich der Leguminosen, Berichte d. 

 deutschen Botanischen Gesellschaft, Vol. X, 1892, pp. 494-513. 



