132 THE ROOT 



pai-eiR'hyma Avhicli resembles the pilh region of tlie stem. Al- 

 though the number of phloem and xylem groups in a given root 

 is equal, variations occur between the roots of the same i)lant as 

 well as between different species (ef. p]sau, 1941, p. 452, Table I). 

 There appears to be no satisfactory explanation for the incon- 

 stancy in the number of phloem and xj'lem strands so character- 

 istic of the roots of some angiosperms. Depending upon the 

 number of primary xylem groups, the stele is described as diarrh 

 (2), triarch (3), tetrarch (4), pcntarch (5), etc. The stele in 

 monocotyledons usually consists of many alternating xylem and 

 phloem groups and hence is designated as pohjarch. 



In further contrast to the stem of seed plants, the primary 

 xylem of the root is exarch. This means that the radial matura- 

 tion of the tracheary elements from jirovascular tissue occurs in 

 the centripetal direction. Ilence the protoxylem lies at the outer- 

 most edge of each xylem strand, next to the pericycle, while the 

 metaxylem is situated towards the center of the stele. Since 

 exarch primary xylem is only found in the stems of the lower vas- 

 cular plants (e.g., the Psilopsida and Lycopsida), the root of seed 

 plants has been regarded by some anatomists as a "conservative" 

 or "primitive" organ (cf. Jeffrey, Ch. XII). Much work needs to 

 be done on the comparative histogenesis of the primary tissues in 

 the root. In several recent studies, Esau (1940, 1941) has made 

 important contributions to our knowledge in this direction. It is 

 interesting to note that her observations indicate that the phloem 

 begins to differentiate nearer the apical meristem than does the 

 xylem. In the protaphloem of carrot, for example, "the sieve- 

 tubes complete their differentiation about 300 microns from the 

 apex of the root" whereas the first protoxylem elements "show 

 secondary walls approximately 1 millimeter from the root apex, 

 but do not lose Iheir jirotoplasts through another millimeter of 

 the root." 



(c) Origin of lateral root>i. Branching of the stem normally 

 originates from lateral buds which develop at or near the shoot 

 apex from superficial cells or cell layers. In marked contrast, 

 the hranchinej of fhe root 7.s stricthf endogenous. The origin of 

 lateral root primordia usually occurs through the renewed growth 

 and division of certain cells in the pericych of the stele distal to 



