GEOWTH AND CELL DIVISION 



61 



For this the root tip furnishes the best material both for obser- 

 vations of growth in a living object and for the study of its more 

 detailed features as revealed in its cellular organization. 



Roolcap-* 



Permanent 

 Maturing 



Elongating 

 Meristem 



GROWTH AND STRUCTURE OF THE ROOT 



The elongation of the root is easily observed in living roots 

 by selecting the first or primary root, which springs from a seed 

 during its early stages 'of germination. In order to locate the 

 exact region in which growth in 

 length takes place the tip of the 

 root should first be marked with 

 fine lines, evenly spaced, for 

 several millimeters back of the 

 apex of the root tip. In Fig. 31 

 these lines have been made at 

 intervals of two millimeters in 6, 

 and the separation of the lines 

 in c indicates where the root 

 grew in length for the first 

 twenty-four hours after it was 

 marked. It will be noted by 

 consulting the figures that the root elongated in the second and 

 third millimeter spaces back of its apex, and that the first milli- 

 meter space at the very apex of the root tip and the millimeter 

 spaces just back of the elongating area have remained unchanged. 

 In the first millimeter or two, at the apex of the root, two dis- 

 tinct groups of cells exist, with which we shall be concerned as 

 we proceed to study the method of growth in the root tip. One 

 of these is apparent in living roots as a light-colored, conical 

 mass of cells constituting the apical point of the root. This 

 light-colored point is the protective rootcap, which incloses a 

 disk of cells, not more than a millimeter in length, which is 

 invisible in fresh roots but is easily recognized in sections of 

 growing root tips. This latter disk of cells is the meristem, or 

 dividing cell zone, which plays a very prominent part in root 

 growth, since it furnishes most of the new cells for the rootcap 



FIG. 31. Growth in length of the 

 root of corn 



The separation of the millimeter spaces 

 indicates the place of greatest elongation 



