COMBINATIONS OF CELLS. TISSUES. 509 



Woody fibre is the main constituent of the trunks of Dicotyledons ; its 

 cells are" mostly of rectangular section, and the walls become greatly 

 thickened with age. Liber, the tibrous substance of the bark of Dico- 

 tyledons, a principal constituent in the fibre-vascular bundles of Monoco- 

 tyledons (fig. 521), and of the fibrous husks of fruits, &c., is composed of 

 very long cells, whose membranes are of a peculiar toughness, even when 

 greatly thickened ; their section is commonly roundish (fig. 529) or hexa- 

 gonal. The peculiar tenacity of the vegetable fibres, Flax, Hemp, &c., 

 arises from the forms and mode of union of the liber-cells of which they 

 consist ; the " grain " of wood is likewise determined by the direction of 

 the long axis of the prosenchymatous cells of which it is composed. 



Conducting cells are long, cylindrical, thin-walled cells, placed one over 

 the other, and not tapering at the ends, and are supposed to be channels 

 for the passage of the nutrient fluid. 



Collenchyma is a substance formed especially at the points of 

 contact of cells. It is of a cartilaginous or horny texture, its cells 

 becoming greatly thickened by secondary layers of a substance 

 softening or swelling up in water, or on the addition of weak sul- 

 phuric acid. It never becomes lignified. 



The lamination of the cell-walls is often invisible until after macera- 

 tion ; so that the tissue looks like a mass of homogeneous substance, exca- 

 vated into cavities, or like a collection of cells with abundant intercellular 

 substance. A solution of chromic acid also serves to show the laminated 

 structure ; but if used too strong, it dissolves the intercellular substance. 

 The outer portion is not coloured by Schulze's solution or anilinj but 

 the inner portion next the cell-wall is tinted blue with iodine. This 

 tissue occurs in the rind of many herbaceous plants, as Chenopodiaceae 

 (tigs. 550 & 557), Cucurbita, Nymphcea (tig. 535), and the pith and medul- 



Fig. 556. Fig. 557. 



Fig. 556. Transverse section of collenchyma-cells of the stem of Beet: a, thickened cell-wall 



Magn. 400 diam. 

 Fig. 5-37. Section of the junction of four cells (a) of fig. 556, treated with hydrochloric acid : a, 



lamina bounding the cavity of the cells ; b, swollen secondary layers ; c, primary 



membrane. Magn. 400 diam. 



lary rays of the species of Astragalus (forming " tragacanth ") ; and to 

 the same head may be referred the substance of fleshy endosperms (tig. 

 558), and also the cartilaginous thallus of the larger Algje. 



