382 EFFECT OF MOLAR AGENTS UPON GROWTH [Cn. XIV 



consisting of only a single cell, have the power of forcibly 

 boring their way between the fibres of the paper. This phe- 

 nomenon is most easily accounted for on the ground of response 

 to contact. 



5. Animals. - - The phenomenon of thigmotropism is exhib- 

 ited among animals in the power of growing along an irregular 

 substratum. Thus the stolons of hydroids, Bryozoa, and some 

 compound ascidians, upon once gaining the surface of an appro- 

 priate substratum, cling closely to it without reference to the 

 direction, up or down, which the substratum may take. It is 

 not necessary that the surface should be a rough one ; the sur- 

 face film of water may incite the response. More striking is 

 the direction of growth of regenerating hydroid stems, accord- 

 ing to the observations of LOEB ('91, p. 18). This experi- 

 menter found that when pieces are cut from a stem of Tubularia 

 and are so placed in a cylindrical glass vessel that the lower 

 end lies buried in the sand while the upper end is in contact 

 with the side of the vessel, then, regeneration of the hydrartth 

 occurring, the new growth is perpendicular to the surface of 

 the vessel, i.e. horizontal. This direction is independent of the 

 direction of light rays, since it occurs at all parts of the circum- 

 ference of the vessel ; nor is it a response to gravity, since the 

 stems normally assume a vertical position. The phenomena 

 are, in so far, exactly like those in plants. 



The experiments made upon plants suggest a series which 

 might be made upon animals. Do stolons exhibit nutations to 

 aid in the finding of the solid substratum ? What substances 

 call forth the thigmotactic response ? Will a momentary irri- 

 tation cause turning? The field of thigmotropism, like that of 

 the other tropic phenomena of sessile animals, is an imperfectly 

 worked one. 



6. The Accumulation of Contact-Stimulus and Acclimatization 

 to it. These two phenomena accompanying contact-stimu- 

 lation must be alluded to. PFEFFER ('85, p. 506) pointed out 

 that a soft blow which calls forth no response when given 

 once, will produce bending when given repeatedly for several 

 minutes. The basis for an explanation of this phenomenon is 

 given by an observation of DE VRIES ('73, p. 307), who with- 

 drew the support from a tendril which had already turned 



