272 INTERNAL FACTORS IV. 2 



he proceeds to illustrate his meaning by referring to the influence 

 of light in determining whether the sexual organs shall be 

 formed on the upper or under side of a fern prothallus, to the 

 dependence of the fate of indifferent buds of plants upon external 

 conditions, to the formation of stems on the upper, roots on the 

 lower side of a horizontal stolon of Antennularia, as demonstrated 

 by Loeb, and to certain alterations in the character of epithelia 

 and other tissues which are brought about by pathological 

 conditions. 



For a more detailed elaboration of this very fruitful idea we 

 are indebted to Herbst. Herbst classifies organic reactions to 

 stimuli as either directive or formative. The former are of two 

 kinds, tactic when the response is some locomotion of a freer 

 body, tropic when the movement exhibited in response is a 

 movement of growth. The reactions may be further classified 

 according to the nature of the stimulus, gravity, light, heat, 

 electricity, chemical substances, currents of water, the contact 

 of a solid surface. Thus we have positive and negative helio- 

 tropism, galvanotaxis, geotropism, galvanotropism, thigmotaxis, 

 and so on, and many examples of all of these are cited. These 

 are mostly familiar facts, but some, for their special interest, 

 may be quoted here. Thus the hydroid Sertularella in one-sided 

 illumination produces stolons instead of hydranths, the tubes of 

 the Polychaet Serpula grow upwards towards the light (or 

 against gravity), an inverted stock of Sertularia produces a polyp 

 sympodium at the upper originally lower end, the stems of 

 Antennularia are negatively, the stolons positively geotropic, 

 spermatozoa swim to solid bodies, and stolons of Hydroids attach 

 themselves to some foreign object. 



Speaking generally, a specific reaction depends not merely on 

 the nature of the stimulus, but on the structure of the reacting 

 body itself. Further, the degree of sensitiveness to the stimulus 

 is not constant. It varies at different periods of life, under the 

 influence of external conditions such as temperature, and with 

 the strength of an already existing stimulus (Weber's Law). 



Now many of the events of ontogeny, as we have seen, resolve 

 themselves into movements of the parts of the organism 

 movements of free parts, locomotion, and growth movements 



