246 - Mulficellulor Plants 



ENDODERMIS 



ROOT HAIR 



SIEVE TUBES, PHLOEM 



STORAGE PARENCHYMA 



PASSAGE CELLS 



PERICYCLE 



DUCTS, XYLEM 

 EPIDERMIS 



of the cortex. Thus water continues to pass 

 inward through the root tissues, toward the 

 ducts in the vascular cylinder. The ducts con- 

 tain the sap of the plant. This solution, be- 

 cause of its content of sugars and salts, is 

 distinctly hypertonic, and tends to absorb 

 water from the surrounding tissues. The 

 water entering the sap generates a pressure 

 in the ducts; and this root pressure forces 

 the sap to flow upward through the plant. 

 At the top ol the plant, much water con- 

 stantly escapes from the leaves (p. 250), and 

 in the leaves ne\v sugars ate being synthe- 

 sized and sent downward to the roots. These 

 processes are necessary to maintain the hvper- 

 tonicity of the sap and to foster a further 

 absorption of water (see p. 252). 



The root also absorbs inorganic salts from 

 the soil, although this is a relatively slow 



Fig. 13-11. Diagrammat : c cross section of a root, in the 

 root-hair zone. Note how soil water enters the xylem 

 through the intervening tissues. 



process. Probably roots are able to augment 

 their salt absorption by energy-expending 

 processes, but these processes are not well 

 understood. Ordinarily, inorganic salts are 

 absorbed in very small amounts, and conse- 

 quently large volumes of sap must flow to the 

 upper parts of the plant to supply the me- 

 tabolic needs ol the tissues (see later). 



The Stem and Its Functions. The exten- 

 sivelv branching stem system of the typical 

 land plant has two primary functions: (1) it 

 supports the leaves in a position that assures 

 a maximum of photosynthesis, ami (2) it 

 conducts materials upward and downward 

 between the leaves and the roots. 



The vascular functions of the plant are 

 performed mainly bv the ducts, or vessels, 

 and sieve tubes. These vascular elements al- 

 ways occur in the form of organized strands, 



