36 BORDERED PITS 



admit of stretching of tlie wall, which would otherwise be 

 ruptured. 



The first-formed vessels of the wood, which differentiate a 

 short distance behind the growing point, exhibit continuous 

 spirals or disconnected rings of thickening {spiral and annular 

 vessels, Fig. 17, A, B). These arc deposited on the inner side 

 of the original vessel wall, to which, however, they are only 

 attached by a narrow connection, as can be seen in optical 

 section (Fig. 17, C). As the organ elongates the spirals are drawn 

 out like a spring, and the rings become more widely separated, 

 by the gradual stretching of the unthickened part of the wall. 

 If, however, growth in length is very considerable (as, for instance, 

 in many Monocotyledons), complete rupture may ultimately take 

 place, so that in the mature plant only an irregular canal re- 

 mains to indicate where the firstTormed vessels were situated 

 (Fig. 40, ^.f.). 



The vessels formed after completion of growth often exhibit 

 reticulate thickening (Fig. 17, G), which appears as a more or 

 less irregular network of ridges deposited on the inner side of 

 the original wall. In many cases careful examination shows 

 that the meshes of this reticulum possess the characters of the 

 bordered pits about to be described. 



Such bordered pits are very common on the vessels in the 

 older wood. An examination of the surface of the vessel wall 

 under the high power of the microscope shows that each pit is 

 provided with a broad harder (Fig. 17, E, h. ; also Fig. 18, B, h). 

 This is either circular in outline, or more commonly polj'gonal 

 as a result of dense crowding, the actual aperture of the pit 

 appearing round (Fig. 18, B, />) or more or less slit-shaped. In 

 section the border is recognised as a dome-shaped cover formed 

 by the thickening of the \\all, which around the pit arches awaj' 

 from the midcUe lamella (i.e. pit-membrane) ; the top of this 

 low dome is perforated Ijy the aperture leading into the pit 

 (cf. also Fig. 197, D, p. 341). Where two vessels adjoin one 

 another, such a perforated dome occurs on either side of the 

 common wall, so that these bordered pits coincide. The pit- 

 membrane possesses a central thickened area, the torus, which, 

 owing to its modified character , is impervious alike to air and water. 

 When the pit-membrane, which is easily chsplaced, is forced 



