ABSORBING TISSUES 



131 



well as conductive tissue. {Fig. 118.) In Flowering Plants, 

 although tracheids are present, the water-conducting tissue is 

 composed mainly of cells which fit 

 together end to end and thus form a 

 continuous series. The end walls of 

 the cells of the series are resorbed and 

 thus are formed continuous tubes, 

 called ducts, vessels, or tracheae, the 

 last name referring to their resem- 

 blance to the human trachea. In the 

 phloem, the main conductive tissue is 

 composed of the sieve tubes, which are 

 so named because of the perforations 

 in their walls. UnUke tracheae, which 

 have thickened woody areas in their 

 walls, sieve tubes have thin cellulose 

 walls and retain their protoplasm. 

 With the sieve tubes usually occur 

 thin-waUed elongated cells, known as 

 companion cells, and parenchyma cells, 

 both of which aid in conduction. 



Absorbing Tissues. — In the higher 

 plants, where the plant body is dif- 

 ferentiated into roots, stem, and leaves, the roots are especially 

 devoted to absorption. In case of soil roots, the root hairs, 



Fig. 118. — Tracheids from 

 wood of Pine, showing the 

 tapering ends and the bor- 

 dered pits (p). After Cham- 

 berlain. 



Fig. 119. — A, root hairs, the absorptive structures of roots, as they appear 

 in a surface view of the tip of a root. B, cross section of a root, showing that 

 the root hairs (h) are projections of the epidermal cells (e). 



