IXDIMDUATIOX AND REPRODUCTION 



221 



formation of a hydranth at the basal end of pieces of the stem of 

 Tubularia under experimental conditions (Fig. 76, p. 211) is simply 

 the same reproductive process which occurs in nature, except that 

 under the experimental conditions it occurs in a shorter length of 

 stem because the rate of metaboHsm is lower. In Planaria and 

 other fiatworms which undergo fission the body attains a certain 

 length and then the posterior region becomes a new zooid, as de- 

 scribed in chap. vi. The length which the individual attains 

 can be widely varied and controlled by experimental conditions 

 which affect the rate of metabolism (Child, 'iic). 



Fig. 95. — Reproduction of new plants from runners in the strawberry. From 

 Seubert, '66. 



In plants similar relations are of very general occurrence. In 

 the strawberry plant, for example (see Fig. 95), the runner attains 

 a certain length before the growing tip gives rise to a new plant, 

 but by cutting off or inhibiting the metaboHsm of the growing tip 

 of the parent plant the development of a new plant at the tip of 

 the runner can be induced at any time. These few cases will serve 

 to call to mind many others among both plants and animals in 

 which a spatial factor and a Hmit of effectiveness of the dominance 

 of the apical or head-region is evident. 



Within the limits of the individual organism the same factor 

 appears in the length and position of various parts, and it has been 



