GRO WTH. 



71 



Fig. 66. 



creasing in size, and a third in which the cells have attained 

 their full size and maturity. The time inclusive of the forma- 

 tion and enlargement of a cell is termed its grand period of 

 growth. In the case of an organ, this period includes the times 

 from the formation of all of its cells to their maturity. All of 

 the cells are not formed at the same time and do not reach 

 maturity at the same time. The portion containing the cells 

 which are enlarging most actively is designated the zone of 

 inaximum growth. This zone is constantly changing its posi- 

 tion, as may be seen in the following experiments. 

 EXPERIMENT 89. 



ZONE OF MAXIMUM GROWTH OF ROOTS. 



Select a healthy seedling of Pea, Bean, or Squash with a root- 

 let 2 cm. in length. With a pointed 

 camel's-hair brush mark off ten in- 

 tervals I mm. apart. Place the seed- 

 ling in a thistle-tube as in Experiment 

 87. Set where it may receive an equal- 

 sided illumination. In twenty-four 

 hours observe the length of the inter- 

 vals. The fourth, fifth, sixth, and 

 seventh from the tip will be found to 

 have elongated much more than any 

 of the others. Twenty-four hours 

 later the terminal division will have 

 partaken of this elongation, showing 

 that the zone of maximum growth 

 moves steadily_toward the tip. Now 

 follow the growth of the second inter- 

 val. At the beginning of the experi- 

 ment it elongates somewhat slowly at 

 first, then more rapidly, until it is grow- 

 ing more rapidly than any other por- 

 tion of the root. Its rate then de- 

 creases until it finally ceases. In the 

 mean time the next interval toward the 

 tip begins to increase in rapidity, 



Seedlings of Pea. (Sachs.) 

 Showing zone of maximum 

 growth. 



