GROWTH. 163 



tened organs and different sectors of cylindrical ones, bring 

 about a bending of the entire organ from one side to another. 

 These curvatures produce nutation, and will be further de- 

 scribed under movements. (See ^241.) 



226. Duration. — Even when the external conditions of 

 growth are kept as uniform as possible, growth does not con- 

 tinue for an indefinite time. Having passed through the 

 phases above named, it ceases, no matter how favorable the 

 external conditions. Yet some organs, even after growth has 

 ceased, may resume it, under certain circumstances. Thus, 

 the leaf cells which have long since ceased to divide may re- 

 sume the power of division in the neighborhood of a wound, 

 and by division and the growth of new cells may form a scar 

 covering the wound. The formation of fruits of the seed 

 plants is also a case of resumption of growth after an appro- 

 priate stimulus. (See ^ 306.) 



227. Summary. — Growth is permanent change of form 

 and increase in size. Every part passes successively through 

 three stages of growth, the first marked chiefly by the forma- 

 tion of new cells, the second by the enlargement of cells al- 

 ready formed, and the third by the acquisition of mature 

 characters by these cells. The second stage is the stage of 

 visible and measurable growth. Only a very short part of 

 the root and a limited region of the stem is growing in length. 

 During the stage of enlargement the growth is not uniform. 

 The rate varies oa account of internal (unknown) and exter- 

 nal (known) causes. Among the latter are light and heat. 

 Light generally retards growth in length, but promotes the 

 growth in area of leaves and other broad parts. It may also 

 produce changes in structure as well as form. Rising tem- 

 perature (up to a limit) hastens growth ; falling temperature 

 retards it. Combined effects of light and heat produce a 

 daily fluctuation in growth. Pressure, amount of water, and 

 oxygen also affect growth. Growth may be resumed by 

 mature parts. 



