I] REACTIONS TO STIMULI 29 



By the addition of staled media to his cultures Balls was able to show that 

 accumulation of staling substances was a limiting factor in the growth of 

 his fungus. 



The products of past metabolism are not the only factors which exercise a 

 negatively chemotropic influence; Miyoshi showed that hyphae tend to grow 

 away from acids, alkalis, alcohol, toxic compounds and certain neutral salts. 



He also brought forward evidence of the positively chemotropic effect of 

 a number of salts and of sugar and other nutritive materials; but his results 

 are vitiated by the fact that he was unaware of the importance of staling 

 substances and accepted, as due to the attraction of the new medium, curva- 

 tures which really depended on the repellant effect of the old. Neverthe- 

 less, positive chemotropism exists and may be observed when the staling 

 substances have been taken into account. 



Thus Graves found that, while all the germ-tubes of Rhizopns nigricans 

 turned from the turnip juice agar on which they had been growing to fresh 

 turnip juice agar, only 60 to 90% turned to fresh plain agar, and, when 

 growth had just begun in the turnip juice agar and staling was consequently 

 less, a smaller proportion of hyphae sought the plain substratum. Positively 

 chemotropic reactions towards cane-sugar and glucose were also demon- 

 strated but they were relatively weak. It may be inferred that the dominant 

 influence governing the distribution of a parasitic fungus in its host, like that 

 governing the distribution of saprophytic forms in culture, is negative chemo- 

 tropism excited by the products of metabolism. 



This inference is borne out by the observation of Robinson that the 

 germ-tubes from the basidiospores of Pnccinia M alvacearum failed to show- 

 any positively chemotropic curvature towards fragments of the host leaf. 



Hydrotropism. Positive hydrotropism is probably effective in vegetative 

 hyphae under appropriate conditions; thus Fulton found that, when spores 

 of various species were grown on gelatine between two perforated mica 

 plates with a relatively moist medium on one side and a relatively dry one 

 on the other, they grew through the perforations towards the moist rather 

 than towards the dry medium; hyphae of Mucor Mucedo grew through per- 

 forations in a mica plate from firm gelatine into water, but hyphae of 

 Rhizopus nigricans, though they grew into the relatively moist gelatine near 

 the holes, turned away from the fluid water below. 



Germ-tubes of Puccinia Malvacearum were found by Robinson to grow 

 from drops of water into the surrounding moist atmosphere, but on gelatine 

 in moist air they tended to penetrate the gelatine. It would seem that the 

 response varies, as is indeed to be expected, with the conditions of the fungus 

 in question, but in some of these and perhaps other tropic reactions, there is at 

 least a suspicion of the negatively chemotropic influence of staling substani es. 



Negative hydrotropism has been described for the sporangiophon ol 



