Plant World Continued 



235 



The final tissues are usually grouped according to their functions. 

 They are : 



Covering or Protective Tissues. (Fig. 136.) Epidermis and 

 periderm. 



Supporting or Mechanical Tissues. (Fig. 137.) All fibrous tissues, 

 such as wood and bast fibers, stone cells (sclerenchyma), polygonal cells 

 with very thick cellulose walls, especially thick at the angles (collen- 

 chyma) which take the place of woody tissue in annual herbaceous or 

 green stems, fruits, seeds, and leaves. Collenchyma is usually associated 

 with the fibrous tissues in the midrib of leaves. 



5D900000 1 



Fig. 136. Epidermal Tissues. 



A, Sectional views of Leaf Epidermis. 1. Upper epidermis, Ficus leaf. 2. Lower 

 epidermis, Ficus leaf. 3. Upper epidermis, Eucalyptus leaf. 4. Epidermis of Pine 

 leaf. 5. Upper epidermis, Orange leaf. 6. Upper epidermis, Geranium (Pelargo- 

 nium), leaf. E, epidermis. H, hair. 



B, Surface views of Leaf Epidermis. 1. Hepatica leaf (wavy walls). 2. Chima- 

 philla leaf (beaded walls). 3. Henbane leaf (wavy and striated walls). 4. Senna 

 leaf (angled cells). 5. Convallaria leaf (beaded walls). (From C. W. Ballard's 

 "Vegetable Histology," courtesy of John Wiley & Sons.) 



Absorption Tissues. (Fig. 138.) Root-hairs for liquids, and 

 stomata (openings usually on the underside of leaves surrounded by two 

 sausage-shaped guard-cells) and lenticels (openings in the periderm or 

 corky coverings of mature woody plants.) 



Conducting Tissues. (Fig. 139.) This consists of ducts known as 

 tracheae if they are continuous tubes formed by the absorption of the 

 connecting cell's end-walls, and there is a disappearance of the cell con- 

 tents. These tubes may be pitted (when there are numerous pores 

 through the cell wall), reticulate (when the lignin laid down on the 

 inner side of the cell wall is in the shape of a network), scalariform 

 (when the non-lignified portions of the cell walls form long narrow slits 



