92 PART II. ANATOMY AND HISTOLOGY. [ 28 



of a multicellular body, it occurs sometimes in internal tissue 

 (e.g. endodermis). 



4. Sdcrenchymatous tissue, or sclerenchyma, consists .typically 

 of_prosenchymatous cells which lose their protoplasm relatively 

 early, and then contain only water or air, and are distinguished as 

 fibres ; but in., some cases., they retain their protoplasm, and are 

 then distinguished as fibrous cells. The cell-walls are thickened, 

 sometimes so much so as almost to obliterate the cavity or 

 lumen (Fig. 70) ; they are frequently lignified 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. 72). 



Fio. 70. Longitudinal section of the cortex of 

 the Oak. showing 8 short sclerotic cells : / fibrous 

 sclerenchytna ; p parenchymatous cells. ( x 300.) 



Fio. 71. Isolated sclerotic 

 cell from the leaf of Exostemma 

 (Rubiacete). (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 protoplasm) of irregular form 

 (Fig. 71) 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. Nymphseacese, 

 Aroids, Limnanthemum, Rhizophora) they are sometimes called 

 internal hairs ; short, straight cells occur in the secondary bast 

 and cortex of many trees (Fig. 70). 



