6o STIMULATIVE INFLUENCES. 



(I.) as o.oi per cent, in the case of glucose acting on Mncor mui'edo 

 oi^ Rliizopus nii/ricans. On gradually increasing the dose above 

 this minimum limit, a second limit is reached whereat the 

 effect is one of repulsion, not attraction. Thus, in the case of 

 Miyoshi's experiments, the integument of the spores of lihizojnis 

 nif/ricatis just began to be appreciably attracted by a o.i per 

 cent, solution of saccharose, the effect then increasing with the 

 dose, until, at about 15 per cent., diminution commenced; and 

 repulsion could be detected at about 50 per cent. The maxi- 

 mum distance at which the influence of the stimulant could 

 be detected was, in the case of the germ tubes of Rhizopas 

 ni(/n'ra7is and Phycomyces nitens, about twelve to fifteen times 

 the longitudinal diameter of the spores. 



Two different explanations have been put forward as to the 

 nature of the intimate causation of negative chemotropism. 

 Massart (I.) assumes that osmotic action is really in question, 

 and that consequently the repulsion of the fungus from the seat 

 of the stimulus is a result of plasinolysis, due to the high con- 

 centration of the solution. On the other hand, Pfeffer (IV.) 

 and Miyoshi found a series of substances which produce an 

 effect of repulsion in all stages of concentration : e.g. potassium 

 chlorate, alcohol, all the acids so far examined, &c. Consequently 

 both workers hold the opinion that this effect cannot be ex- 

 plained as the result of osmosis (like the case of the other sub- 

 stances), but is attributable to a specific power of repulsion. 



The relation between the strength of the stimulus and the 

 degree of effect produced was investigated by Pfeffer, and found 

 to agree with the general law enunciated by Weber, viz. " The 

 magnitude of the reaction increases in arithmetical progression, 

 as the magnitude of the stimulus increases geometrically." Of 

 course the application of this law is here confined within certain 

 limits, and is therefore excluded when p>ositive chemoti-ojiism 

 begins to turn into negative in consequence of a strongly in- 

 creased concentration of the stimulant. 



Further investigation is merited by the question of how far 

 chemotropic action is concerned in the phenomenon known as 

 rheotropism, the outward sign of which is the adoption by 

 fungoid hyphfe of a definite position with respect to the direc- 

 tion of flow of the surrounding liquid. Jcexsson (I), to whom 

 we are indebted for the first observation on this point, found 

 that the mycelial hyphre of BotrytU grow against the current, 

 but those of Mucor and Phycomyces with the stream. The first- 

 named fungus he termed positively rheotropic, the other two 

 negatively so. 



The tendency of fungoid hyph* to gi'ow towards a positively 

 chemotropic stimulus is manifested not merely when the latter 

 is freely accessible, but also when it is separated from the in- 

 fluenced fungus by a partition. In some species and under special 



