GROWTH. 



i8i 



characterized by permanent and usually irregular thickenings 

 of the wall (figs. 10, n, 52, 58). 



257. Grand period of growth. — The entire duration of 

 growth of an organ is known as its grand period of growth. 

 Corresponding precisely to the phases in cell development, 

 there are three phases in the development of the organ as a 

 whole. Its growth is at first very slow, increasing gradually, 

 and then more rapidly, to a maximum, from which it falls 

 rapidly, and then more gradually, until it ceases entirely. The 

 earliest phase, the embryonal, results in so little elongation 

 that it is scarcely possible to have it recorded by the auxanom- 

 eter. The last phase, that of internal differentiation, is not 



Fig. 183. — Curve representing the rate of growth of an internode >>f crown imperial tor 

 each day during the grand period — in this rase _'*. days. The height ol each \ ertical line 

 where it intersects the curve represents the total growth for the corresponding 24 

 hours. The numbers indicate days. The maximum growth occurred on the 6th day. 

 — After Sadis. 



marked by any elongation. The accompanying curve (fig. 

 [83) therefore represents only the duration and course of the 

 phase of elongation. 



258. Growing region.— The part of any one of the multi- 

 cellular plants, which is growing in length, is limited. The 

 elongating region of a root rarely exceeds a centimeter, and 

 is often not more than one halt a centimeter in length. In 



