40 



THE PLANT CELL. 



(6) CoUenchyma. — This tissue usually occurs just internal to 

 the epi4erinis, in such stems as that of Cucurbita ; the cells com- 

 posing it are in reality the outermost cortical cells, at the angles 



of junction of which the inter- 

 cellular substance becomes con- 

 verted into a material, highly 

 retractile in appearance, which 

 when dry is not unlike dried 

 mucilage. On the addition of 

 water or caustic potash solution 

 it swells up to many times its 

 original bulk, and it may be 

 stained with methylene blue 

 (see Fig. 26). CoUenchyma 

 confers elasticity upon the 



„. _ „ outer layers of the cortex, 



Fig. 25.— Cortical Cells froji the "^ j . . • i 



Young Stem OF Pinm—M), Cell- and, as a protective layer 



wall ; k, chloroplasts lying in the against mechanical shock, must 

 granular cytoplasm ; n, nucleus ; ^^ ^f ^^^ service to the plant. 

 St, starch-granules. , ^ „ , , t • 



(c) Sclerenchyma. — In vari- 

 ous parts of a plant there occur elongated fibres, massed 

 together into bundles or zones of greater or lesser extent; 

 they are to be found in the outer parts of the rhizome 



Fig. 26.— CoLLENCHYMA FROM THE Stem OF Cucurhita (transverse 

 section). — On treatment with dilute caustic potash the intercellular 

 siibstance at the angles of the cells swells up to twice or three 

 times its original volume. 



or petioles of Ferns, 

 bundles in these plants. 



and also surrounding the vascular 

 Each fibre arises at an early stage from 



