27.] 



CHAPTER II. THE TISSUES. 



129 



m 



In most cases the closing-membrane between two opposite pits 

 persists (Fig. 87) ; always whilst the contiguous cells contain pro- 

 toplasm, less frequently when these 

 cells eventually lose their proto- 

 plasm. A good example of the 

 latter case is afforded by the vas- 

 cular cells of the wood (tracheids) 

 of Conifers. Here the pits are bor- 

 dered (see p. 104), and are closed 

 by a membrane (Fig. 88 C) which 

 is somewhat thickened in the mid- 

 dle, the thickened area being called 

 the torus, and is either situate in 

 the middle line between the two 

 pits (Fig. 88 (7) or is forced to one 

 side (B). 



On the other hand it is com- 

 monly the case that the thin pit- 

 membrane undergoes absorption, 

 so that the cavities of the adjacent cells become continuous, and 

 in some cases the entire septum may be absorbed ; in either case 



-Tit 



FIG. 87. Transverse section of the 

 cortical cells of Trichomanes speciosum 

 (x 500). Middle lamella (m) ; ii the 

 secondary layers of the cell-wall ad- 

 joining the middle lamella; I coll- 

 cavity ; t bordered pits which are oppo- 

 site in adjoining cells, but are closed 

 by the middle lamella. 



FIG. 88. Pinus sylvestris ( x 540: after Strasburger). A surf ace- view of a bordered pit; 

 J? bordered pit as seen in a longitudinal tangential section of the wood ; t the torus of the 

 closing-membrane which is thrust to one side of the pit-cavity ; C transverse section of a 

 tracheid showing bordered pits with closing-membranes; TO middle lamella; i secondary 

 layers of cell-wall ; m* thickened middle lamella at the junction of several septa. 



the result is a syncyte (p. 91), of which the most familiar examples 

 are the wood -vessels of many plants, where the transverse septa 

 of a longitudinal scries of cells are more or less completed ab- 

 sorbed, giving rise to a tube ; but the absorption of the wall may 



v. s. B. K 



