78 



PHYSIOLOGY. 



186. The region of elongation. — While the root has elon- 

 gated, the region of elongation is not at the lip 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 4-5 of the milli- 

 meter marks. The 

 root shown in fig. 66 

 was marked at 10 a.m. 

 on July 5. At 6 p.m. 

 of the same 



Fig. 66. 



Root of germinating pumpkin, showing region of 



elongation just back of the tip. 



hours later, growth had taken place as shown in the middle 

 figure. At 9 a.m. on the following day, 15 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. Farther 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 pump- 

 kins, or other plants, at different periods. 



187. 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 spaces 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 ap- 

 proximately at the same distance behind the tip. 



188. Formative region — If we make a longitudinal section of the tip of a 

 growing root of the pumpkin or other seedling, and examine it with the mi- 



