Biology of Higher Plants — Anatomy and Physiology 221 



than the portion of the sieve tube which arises from one cell. The cyto- 

 plasm and nucleus are present in companion cells, while the nuclei of 

 cells which form sieve tubes disappear. The tracheids and vessels of the 

 primary xylem conduct water and other materials from the roots to the 

 leaves. The sieve tubes and companion cells of the primary phloem con- 

 duct to the roots organic materials which they have received from the 

 stems and leaves. The parenchyma of the root is composed of numerous 

 thin-walled cells which lie between the primary xylem and primary 

 phloem. 



All the tissues just described are primary tissues and arise directly by 

 cell division from the embryonic region. In the mature region of roots 

 of dicotyledonous plants (seeds having two cotyledons or seed leaves), 

 certain cells remain undifferentiated, retain their embryonic character, 

 and form cambium. The cambium is bounded by the phloem on the 

 outside and xylem on the inside. Somewhat back of the mature region, 

 the cambium forms a continuous cylinder between the xylem and phloem. 

 The cambium consists of small cubical cells which divide rapidly by 

 mitosis during the growing season. Growth of the cambium causes the 

 root to increase in diameter. Tissues formed by the cambium are known 

 as secondary tissues, which include ( 1 ) secondary xylem, ( 2 ) secondary 

 phloem, (3) medullary rays, and (4) annual rings. The secondary 

 xylem is formed on the inner side of the cambium, while the secondary 

 phloem is formed on the outer side. As the root becomes older, the sec- 

 ondary xylem and phloem occupy most of the stele and form two con- 

 centric regions separated by cambium. In certain places the cambium 

 produces parenchymatous strands which extend radially between the 

 xylem cells toward the center and between the phloem cells toward the 

 periphery of the stele. These radially arranged strands are the medullary 

 rays. The secondary xylem produced by a single year's growth is rep- 

 resented by a so-called annular ring. In the roots of plants which live 

 several years, the formation of new secondary xylem and phloem con- 

 tinues year after year, forming concentrically arranged annular rings. 

 The xylem vessels produced during the early spring are generally large 

 and thin walled, while the xylem vessels formed near the end of the 

 growing season are smaller and thick walled. Hence, each annular ring 

 is composed of two types of xylem vessels described above. 



The functions of roots ordinarily include anchorage of the plant, sup- 

 port of the stem, absorption of water and dissolved materials, and storage 

 of manufactured foods. In some instances special types of roots per- 

 form other more specialized functions. 



