234 



GENERAL BIOLOGY 



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

 derm. 



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

 such as wood and bast fibers, stone cells (schlerenchyma), 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. 



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 ( v/avy 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.) Ducts (tracheae are continuous 

 tubes formed by the absorption of the connecting cell's end-walls, and 

 the disappearance of the cell contents.) 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 which are quite uniform). Such cells are often 

 angled (no others are). Annular (thin-walled tubes with rings of \\g- 

 nified tissue within the lumen of the tube), and spiral (where the lig- 



