78 Growth 



photography, and the situation was analyzed more fully by Goodwin and 

 Stepka (1945; Fig. 4-17) and Goodwin and Avers (1956). Erickson and 

 Goddard (1951) used still more refined photographic methods. The lo- 

 cation and rate of cell division in the root offer problems of considerable 

 complexity. 



There are a number of developmental patterns in the root apices of 

 seed plants. They have been classified into various "types" by Janczewski 

 (1874), Kroll (1912), and Schuepp (1926). These are well described 

 in Esau ( 1953&, p. 116). In the structure of the root apex there is ob- 

 viously less uniformity than in that of the shoot. 



The development of the apical meristems in both root and shoot from 

 their first appearance in the early embryo has been studied by a number 

 of workers. A typical example is described in Pseudotsuga by G. S. Allen 

 (1947). 



Root tips, with the regions just behind them, were among the first 

 materials to be used for plant-tissue culture (p. 296) and much has been 

 learned through this technique as to the physiology of the root. In many 

 plants, for example, the root cannot synthesize thiamin but depends for 

 this vitamin on a supply produced in the shoot. 



Mention has already been made (p. 41 ) of the work on the physiology 

 of the root meristem by Brown and his colleagues, who determined the 

 changes that take place in the activity of the apical cells in various re- 

 gions, particularly as to growth rate, respiration rate, and protein syn- 

 thesis. In a general discussion of this work, Brown, Reith, and Robinson 

 (1952) show that there is a considerable difference in the composition 

 of the proteins at different distances from the apex. Jensen (1955) has 

 also made a biochemical analysis of the cells near the root tip in Vicia 

 faba. 



Clowes (1956, 1958) discovered between the active meristematic re- 

 gion and the root cap a cup-shaped group of cells, the quiescent center 

 (Fig 4-18), which from their appearance divide rarely. He reports that 

 these cells synthesize DNA more slowly than do the surrounding ones. 

 They presumably have some specific metabolic function. Jensen and 

 Kavaljian ( 1958 ) have made a census or cell divisions in the root tip of 

 Allium cepa. They found a definite apical initial region where there are 

 few divisions and agree with Clowes that these have a low DNA content. 

 Cell division is slower to start in the axial than in the peripheral region of 

 the tip. They report a very definite daily periodicity in division, with a 

 maximum about noon. 



The growth-substance relations of growing roots have received much 

 attention (p. 391). Auxin and various synthetic substances stimulate the 

 initiation of root primordia but usually check the later growth of the 

 root. Auxin tends to be basipetal in its flow, a fact that helps to account 



