THE ANATOMY OF THE ROOT 



53 



Because of the definite manner in which the lateral roots originate 

 with respect to the protoxylem points of the primary stele, the 

 number of rows of lateral roots is commonly equal to, or twice the 

 number of, the protoxylem points, so that in a tetrarch root there 

 are commonly four vertical rows of laterals, in a triarch root, 

 three, etc. In diarch roots, the laterals are frequently initiated 

 at a slight angle to the protoxylem point, and for this reason in 

 roots such as Apium, Solanum, and Lycopersicum four rows of 

 laterals arise. In many grasses, the lateral roots are initiated at 

 points in the pericycle outside the primary phloem groups. 



Adventitious Roots. — In addition to the development of 

 lateral roots from young root axes, they arise from other organs of 

 the plant, especially the stem. Such roots are called adventitious or 

 adventive. In the most restricted sense, the term is not ordinarily 

 applied to roots arising from other roots, even though they are 

 produced late in ontogeny and in the same manner as those which 

 arise from the stem. The term is applied, in a broader sense, to all 

 roots which do not arise from the pericycle and adjacent tissues in 

 typical acropetal succession at about the time when secondary 

 thickening of the axis has been initiated. On this basis, roots 

 formed late in ontogeny from the peripheral tissues of woody or 

 fleshy roots and those arising as a result of injury to a mature root 

 axis are regarded as adventitious. 



The organization of the root meristem and the ontogeny of the 

 adventitious root are similar in all respects to that described for 

 the lateral root; but the point of origin of such roots deserves 

 further consideration. In general, the origin of adventitious 

 structures, including both roots and shoots, requires the presence 

 at the point of origin of potentially meristematic tissue. For this 

 reason, adventitious roots frequently arise from intercalary meri- 

 stems at the bases of internodes, from the meristematic tissue 

 which commonly occurs at the axils of leaves, from lateral meri- 

 stems such as the cambium and pericycle, from ray parenchyma; 

 and, less frequently, from the parenchymatous tissue of other 

 regions. 



In the case of roots arising from stems, Priestley and Swingle 

 (9) have noted that the tissue from which the root originates is 

 determined in part by the age of the stem from which the adventi- 

 tious root arises. In young stems, they are usually pericyclic in 

 origin, arising from a group of cells rather than from a single cell. 



