GROWTH. 187 



period, but also minor variations independent, so far as 

 known, of external conditions, which are therefore called 

 spontaneous variations. Irregular variations occur from hour 

 to hour in the course of the day. Regular spontaneous 

 variations, also, occur in various organs, particularly in the 

 tendrils of climbing plants, and in the leaves of flowers and 

 buds. These regular variations, which affect different sides 

 of bilateral 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 described under movements. (See ^[ 283.) 



267. 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, provided they are affected by 

 suitable stimuli. Thus, the leaf cells which have long since 

 ceased to divide may resume the power of division in the 

 neighborhood of a wound, and by division and the growth 

 of new cells may form a callus covering the wound. The 

 stimulus following fertilization also induces growth in parts 

 adjacent to the egg, as is most strikingly shown in the 

 formation of fruits of the seed plants. (See ^f 404, 409.) 



