388 EFFECT OF MOLAR AGENTS UPON GROWTH [Cn. XIV 



tain radicles are sensitive to the impact of water, so that they 

 turn in such a way that the irritant shall affect their two sides 

 equally, and shall be directed against the tip. While the pre- 

 cise part of the radicle which is sensitive to the current has 

 not been determined, the responding part is in the region of 

 greatest growth, and the response is such that the root becomes 

 concave towards the irritant. 



Among animals, a response which probably belongs to this 

 category has been observed by LOEB ('91, p. 36) in the grow- 

 ing stem of the hydroid Eudendrium. In a vessel of water in 

 which all other hydranths turned towards the light, there was 

 one which rose near the cloacal opening of an ascidian, from 

 which strong currents of water passed. This individual was 

 turned with its concave side towards the impinging current. 

 It is probable that the current had induced a rheotropic 

 response. 



SUMMARY OF THE CHAPTER 



The rate of growth of the entire organism or of its organs 

 is accelerated by contact, as in the stolons of hydroids ; by 

 rough movement, as in bacteria ; by cutting when accelerated 

 growth occurs along the cut surface. Growth in length 

 may be retarded by pulling, as in stems and roots. Many 

 organisms show themselves very sensitive in their growth to 

 mechanical irritation. 



The direction of growth is determined by external agents, 

 acting as irritants, in the cases of the twining dodder, tendrils 

 in general, roots, the hyphse of Phycomyces, the rhizoids of 

 hepatics, and the hydranths of various hydroids. Wounding 

 may cause a turning from the wounded side in the radicle of a 

 seedling, and radicles and hydroids may even grow so as to 

 face the current. 



The sensitiveness to contact may be excessively great, a 

 swinging cotton rider weighing 0.00025 mg. causing a tendril 

 to bend. This sensitive region varies from near the tip in 

 roots to some distance behind the tip in the case of tendrils. 

 The response may occur after seconds, as in Cuscuta, or after 

 hours, as in traumatropism of radicles. The bending usually 

 reaches a maximum at the region of greatest growth. The 



