Nutrition of Multicellular Plants - 243 



cellular material or space — forming a layer 

 that is only one cell deep. Epidermal cells 

 vary in form in the different parts of the 

 plant. In the leaf (Fig. 13-7), where the epi- 

 dermis protects the internal tissues from los- 

 ing too much water, the outer walls of the 

 cells are thick and cutinized. Leaf epidermis 

 is also unusually transparent, allowing light 

 to pass through to the green tissue inside 

 the leaf. Even in the leaf the epidermal cells 

 do not possess chloroplasts. In the root, where 

 the epidermis has an absorptive function, the 

 cell walls are relatively delicate; and in the 

 zone where absorption is most active, the 

 epidermal cells of the root possess slender 

 outgrowths, the root hairs. 



3. Green Parenchyma, or Chlorenchyma. 

 Parenchyma tissues are not highly special- 

 ized; that is, the cells resemble meristem cells 

 in greater or lesser degree. Green parenchyma 

 possesses chloroplasts (Fig. 13-16), and photo- 



synthesis is the main function. Green paren- 

 chyma cells are usually thin walled and 

 loosely packed. This arrangement provides 

 a continuous system of intercellular spaces 

 and facilitates the exchange of gases during 

 photosynthesis and respiration. Green paren- 

 chyma is localized mainly in the leaves, al- 

 though some chlorenchyma is usually present 

 near the surface in younger stems. 



4. Storage Parenchyma, or Colorless Paren- 

 chyma. Storage parenchyma resembles green 

 parenchyma, except that the cells are gen- 

 erally larger and lack chloroplasts. The main 

 function of colorless parenchyma is to store 

 organic substances (chiefly starch); but the 

 loose arrangement of the cells facilitates 

 respiration in the stem and roots (Fig. 13-8). 



5. Strengthening Tissues, (a) Sclerenchyma: 

 This loosely defined tissue consists of elon- 

 gate cells with very thick cell walls. In some 

 mature sclerenchyma all the protoplasm may 



SPIRAL DUCT 



PITTED DUCT 



; — WOOD FIBERS 



XYLEM 



PARENCHYMA 



-PITH 



SIEVE TUBE 



PHLOEM 



PARENCHYMA 



BAST FIBERS 

 ENDODERMIS 



COLORLESS 

 PARENCHYMA 



GREEN 

 'PARENCHYMA 



> XYLEM 



-CAMBIUM 



> PHLOEM 



y CORTEX 



-EPIDERMIS 



Fig. 13-8. Tissues of the higher plants as shown in a diagrammatic section of the stem of an 

 angiosperm. For convenience of labeling, the stem has been placed horizontally rather than in the 

 normal vertical position. 



