326 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



obtain, if possible, this substance, which we may for convenience 

 call the "staling substance," free from mycelium; and determine 

 whether, if incorporated in a sporeless layer, it would repel the 

 hyphae from growing into that layer. 



It was found impossible to grow the fungus luxuriantly enough 

 in sugar solutions (probably on account of the lack of nitrogen) 

 to produce an adequate amount of the staling substance. But 

 turnip juice, pressed from autoclaved white turnips, was finally tried 

 with excellent results. Previously sterilized flasks containing 

 this, and inoculated with Rhizopus spores, produced a luxuriant 

 growth in a few days. After about a month, the juice was poured 

 off and freed of any spores and mycelium by centrifuging. The 

 staled juice had a sour, malt-like odor, and showed an acid reac- 

 tion to litmus. By adding this juice to the agar for the sporeless 

 layer, and using the fresh juice for the layer with spores, no turning 

 of the hyphae resulted. In previous experiments where fresh 

 instead of stale juice had been used, but with the other conditions 

 the same, the turning had invariably been practically loo per cent 

 (cf. C of TABLE I.). Furthermore, by evaporating the staled juice 

 under reduced pressure at laboratory temperature to one half 

 volume, or double strength, the hyphae were caused actually to 

 turn away from the vicinity of the holes. 



The theory that the hyphae excrete some substance or sub- 

 stances which produce a negatively chemotropic effect, is l^ere- 

 fore fully proved, and the chief evidence may be summarized as 

 follows: 



1. According to the strength of concentration of the staling 

 substance, the hyphae show no turning toward, or may turn away 

 from, a layer containing this. 



2. When an approximately equal amount of mycelium occurs 

 in two layers, each composed of the same medium, no turning 

 from one layer to the other results. 



3. The hyphae always show a marked turning from the medium 

 in which they are growing to a medium without hyphae, pro\idcd 

 the latter does not contain their staling substance or other nega- 

 tively chemotropic substances. 



Experiments with fresh turnip juice. — After the existence and 

 nature of the negati\e chem()tn)i)ism was settled, it was easier 

 to search lor a i)ossil)lc positixe cluMiiotropism. For, without 



