GROWTH. 



103 



177. The region of elongation. While the root has elon- 

 gated, the region of elongation is not at the tip of the root. It 

 lies a little distance back from the tip, beginning at about 2mm 

 from the tip and extending over an area 

 represented by from 4 to 5 of the millimeter 

 marks. The root shown in fig. 66 was 

 marked at 10 A.M. on 

 July 5. At 6 P.M. of 

 the same day, 8 hours 

 later, growth had taken 

 place as shown in the 



Fig. 81. 



Root of germinating pumpkin, showing region of 

 elongation just back of the tip. 



middle figure. At 9 A. M. on the following day, 1 5 hours later, the 

 growth is represented in the lower one. Similar experiments 

 upon a number of seedlings gives the same result : the region of 

 elongation in the growth of the root is situated a little distance 

 back from the tip. Further back very little or no elongation 

 takes place, but growth in diameter continues for some time, 

 as we should discover if we examined the roots of growing 

 pumpkins, or other plants, at different periods. 



178. Movement of region of greatest elongation. In the 

 region of elongation the areas marked off do not all elongate 

 equally at the same time. The middle spaces elongate most 

 rapidly and the spaees marked off by the 6, 7, and 8 mm marks 

 elongate slowly, those farthest from the tip more slowly than 

 the others, since elongation has nearly ceased here. The spaces 

 marked off between the 2-^mm marks also elongate slowly, but 

 soon begin to elongate more rapidly, since that region is becom- 

 ing the region of greatest elongation. Thus the region of 

 greatest elongation moves forward as the root grows, and 

 remains approximately at the same distance behind the tip. 



