STEM OF PINUS. 



141 



be only moderately thin, these will look like narrow spindle- 

 shaped or lenticular hollows in the thickness of the wall ; but 

 if very thin they appear like 

 two pairs of open pliers di- 

 rected towards one another, 

 each pair enclosing a narrow 

 lancet-like arch (cf. Fig. 26, C). 

 The pit is crossed from apex 

 to apex of the two arches, by 

 the delicate closing mem- 

 brane, which thickens in the 

 middle into the torus, or, to 

 be more accurate, the two pit 

 chambers are separated from 

 one another by this closing 

 membrane, which includes a 

 continuation of the middle 

 lamella of the trachei'des (see 



Fig. 27) ; while the small FIG. 51. Cross-section through a portion 



of the stem of Finns sylrestris taken during 



gap between the correspond- winter, with the cambium at rest, and passing 

 in a ends of the iaws of the thro g h autumn wood, cambium and the ad- 



6 joining bast, s, autumn wood : c, cambium ; 



opposing pairs of pliers indi- -, sieve-tubes ; p, bast parenchyma ; k, crys- 

 t J.-L. tallogenous cell of bast ; cu, sieve-tubes which 



cates the Opening Of the } ia . s cea sed to function, and have collapsed ; 

 bordered pit into the cavity '"' medullary ray. 



of the trachei'de. The pit, therefore, broadens rapidly as we ap- 

 proach the closing membrane, and it is this broadening, so as 

 to produce a hollow or chamber in the thickness of the wall, 

 which distinguishes a bordered (or chambered) pit from a simple 

 pit such as we saw in Orniihogalum^ in the Date, and in the 

 cortex of the Cherry laurel (Prunus Lauro-Cerasus) as described 

 on p. 73. Upon the tangential walls of the trachei'des of Pinus 

 sylvestris bordered pits are rarely found, though frequent in the 

 autumn trachei'des of allied trees. In the later trachei'des of 

 the year the bordered pits are much smaller. Between the 

 trachei'des and the cells of the medullary rays, so far as the 

 latter have protoplasmic contents, are very broad, unilaterally 

 bordered pits pits so broad that they occupy almost the entire 

 width of the trachei'de. The border is developed only upon the 

 wall of the trachei'de, not upon that of the medullary ray cell. 

 The closing membrane bulges into the trachei'de ; it has no torus. 

 Between the trachei'des and those medullary ray cells which con- 



