GENERAL CHARACTERISTICS OF GROWTH 323 



Growth Regions. — All the organs of the plant pass through 

 the phases of growth outlined in the preceding paragraphs. They 

 have their period of formation, their grand period of growth and 

 enlargement, and their period of maturation when they settle 

 down into the business of doing the work for which they are pre- 

 pared. 



In aerial roots the growth region may extend over the last five 

 centimeters or more, but in roots growing in the soil the elongating 

 zone is generally restricted to the region at the very tip. If a root 

 is divided into regions 1 mm. long and marked with India ink, 

 it will be noted that the growth increments are in the last centi- 

 meter, the maximum growth being in the region about 3 mm. from 

 the tip. The following figures give the results of a typical experi- 

 ment with Vicia faba roots. The temperature was 20° C, the 

 time 24 hours, and the zones were numbered from the tip upward. 



No. of zone Growth in mm. 



1 1.4 



2 5.9 



3 8.1 



4 3.7 



5 1.4 



6 1.1 



7 0.4 



8 0.3 



9 0.2 

 10 0.1 



Furthermore, each zone goes through a similar grand period 

 of growth. The first two days' growth is comparatively slow, 

 reaching a maximum during the next three days, after which it 

 gradually diminishes to zero. The figures given below illustrate 

 the results obtained in experiments of this kind. 



The stem behaves similarly to the root but with the difference 

 that the growth region extends much further back from the tip. 

 The stem is not surrounded by compact resistant soil, which offers 



