CKIBROSE-CELLS. 



91 



III. Cribrose-cells, Sieve-cells, or Sieve-tubes. 



279. In the inner bark of stems of rlieotyledons with normal 

 structure certnin long cells of peculiar character are found as- 

 sociated with bast-fibres. They 

 are of tubular or prismatic form, 

 and are characterized by tlie pres- 

 ence of circumscribed panels in 

 the walls, in whicli are numerous 

 fine perforations permitting com- 

 munication between contiguous 

 cells. The panels are known as 

 sieve-plates ; tlie perforations, as 

 sieve-pores. These cells consti- 

 tute an essential, though by no *'" ™ 

 means always a conspicuous, element of fibro-vascular bundles. 



Taken collective]3", 

 they may be known 

 as cribriform tissue. 

 By their union end to 

 end they appear like 

 long tubes with the 

 continuity interrupted 

 here and there by cross 

 partitions. These par- 

 titions which separate 

 the individual cells 

 are sometimes nearly 

 horizontal, but more 

 generallj- oblique, as 

 shown in the annexed 

 figures where they 

 mostl}' cut the lateral 

 wall of the cell at a 

 sharp angle. 



280. The walls of 

 cribrose-cells are 

 never lignified ; on 

 the contrary, they are 



Fig. 69. Pinus sylvestris. Face view of radial wall containing two cribrose-plates 

 vrijolly depriveil of callus, "f*. (Janczewski) 



Fig. 70. Pinus sylvestris. Eadial wall of a young tube, face view. The future cri- 

 brose-plates are composed of callus-cylinders, filling the meslies of a cellulose network. 

 ^Y^. (Janczewski.) 



Fig 71. Cribrose-cells in A^'itis vinifera : .^4, transverse anastomosis of two cribrose- 



