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. Unlike 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-walled 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- 
S 
ny 
FO 
Fic. 118. — Tracheids from 
wood of Pine, showing the 
tapering ends and the bor- 
dered pits (p). After Cham- 
berlain, 
ferentiated into roots, stem, and leaves, the roots are especially 
devoted to absorption. In case of soil roots, the root hairs, 
Fic. 119. — A, root hairs, the absorptive structures of roots, as they appear 
in a surface view of the tip of aroot. B, cross section of a root, showing that 
the root hairs (h) are projections of the epidermal cells (e). 
