310 



CONDUCTING SYSTEM 



develop a turgor-pressure of their own, are subject to compression at 

 the hands of the surrounding parenchymatous tissues. 



The structure of bordered pits 159 demands fuller consideration. 

 The cavity of a simple pit remains approximately of the same width 

 throughout. In the case of a bordered pit, on the contrary, the cavity 

 is greatly enlarged in the neighbourhood of the limiting membrane ; 

 since the pits correspond on the two sides of the wall, the result is the 

 formation of a lenticular pit-chamber divided into two equal halves by 

 the limiting membrane. This membrane is not equally thick all over, 

 but bears at its centre a more or less thickened pad or torus, which 

 is slightly wider than the orifice of the pit. Among Conifers the 

 torus is rod-shaped in optical section in the spring-wood, biconvex 

 (lenticular) in the autumn-wood (Fig. 128 a). In contrast to the 



A 



Fig. 12S. 



Structure of the bordered pits of Pinus sylveslris. A. Bordered pits as seen in a 

 tangential section of the wood; a, from spring wood (air-dried); b, from autumn 

 wood ; t, torus. x750. B. T.S. through a tracheide ; the bordered pits are confined 

 to the radial walls, v. A bordered pit in surface view, showing the radial striation 

 of the thin marginal region of the limiting membrane. xlOOO. A and C after 

 Russow, B after Strasburger. 



torus the rest of the closing membrane is exceedingly delicate ; in 

 Conifers this marginal area exhibits very distinct radial striae, which 

 extend right up to the edge of the torus (Fig. 128 c). According to 

 Eussow, this striation probably indicates the presence of slightly 

 thickened strips separated by thinner areas. It should be noted that 

 the limiting membrane has a very strong affinity for certain dyes, and 

 particularly for haematoxylin and eosin. 



The appearance of a bordered pit in surface view varies consider- 

 ably according to the shape of the border and the cavity. Either of 

 these portions may be circular, elliptical or narrowly oblong in outline ; 

 moreover, the border and the orifice of the cavity need not be of the 

 same shape, so that a great number of different combinations may 

 occur. 



As regards the disposition of bordered pits, it may be stated that 

 these structures are generally arranged in vertical or horizontal rows 

 or in spiral series : the spiral arrangement is more particularly charac- 



