CRIBEOSE-CELLS. 



91 



G9 



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



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

 structure certain long cells of peculiar character are found as- 

 sociated with bast-fibres. They 

 are of tubular or prismatic form, 

 and are characterized by the pres- 

 ence of circumscribed panels in 

 the walls, in which are numerous 

 fine perforations permitting com- 

 munication between contiguous 

 cells. The panels are known as 

 sieve-plates ; the perforations, as 

 sieve-pores. These cells consti- 

 tute an essential, though by no 

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



Taken collective] v, 



T-T> 



they ma}' 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 

 generally oblique, as 

 shown in the annexed 

 figures where the}' 

 mostly 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. Finns sylvestris. Face view of radial wall containing two cribrose-plates 

 wholly deprived of callus. - 1 - 1 I 6 - 5 . (Janczewski.) 



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

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

 1J i 65 - (Janczewski.) 



FIG. 71. Cribrose-cells in Vitis vinifera : A, transverse anastomosis of two cnbrose- 



