29.] 



CHAPTER II. THE TISSUES. 



133 



tinguished as fibrous cells. The cell-walls are thickened, some- 

 times so much so as almost to obliterate the cavity or lumen (Fig. 

 92) ; they are frequently ligm'fied throughout, or only partially, or 

 not at all (e.g. bast-fibres of Flax and Hemp); they commonly have 

 simple round pits, or oblique and narrow bordered pits (Fig. 94). 



FIG. 92. Longitudinal section of the cortex of 

 the Oak, showing s short sclerotic cells ; / fibrous 

 sclerenchyma ; p parenchymatous cells. ( x 300.) 



Fia. 93. Isolated sclerotic 

 cell from the leaf of Exostemma 

 (Rubiaceae). (x 300.) 



Sclerenchymatous tissue usually occurs in masses so as to give 

 firmness and rigidity to the various parts in which it is present ; 

 it constitutes, together with . the collenchyma, the mechanically 

 supporting-tissue or stereom of the plant. 



Isolated sclerotic cells (without cytoplasm) of irregular form 

 (Fig. 93) are of frequent occurrence (e.g., in the flesh of Pears, in 

 coriaceous leaves as those of Camellia, Hakea, Olea, etc.) : when 

 these cells project freely into air-cavities (e.g. Nymphseaceae, Aroids, 

 Limnanthemum, Rhizophora) they are sometimes called internal 

 hairs ; short, straight cells occur in the secondary bast and cortex 

 of many trees (Fig. 92). 



In some cases (e.g. Vinca, Nerium, Urtica, Humulus, etc.), the young bast- 

 fibres have been found to become coenocytic in consequence of the multiplica- 

 tion of their nuclei by indirect division. In other cases the sclerenchymatous 

 fibres become septate in consequence of the nuclear divisions being followed by 

 the formation of septa ; such apparently multicellular fibres (not uncommon in 

 the secondary wood of Dicotyledons) are termed chambered fibres. 



5. Tracheal tissue consists of cells which early lose their cyto- 

 plasm; their cell-walls are generally, but not always, lignified, and 



