342 EFFECT OF CHEMICAL AGENTS [Ch. XI 



ures in a dark, moist chamber ; in both cases, the chemotropio 

 responses occurred. That contact was not the determining- 

 factor was shown by placing above and below sheets of perfo- 

 rated collodion a layer of 5% gelatine, deprived of calcium 

 salts, which are attractive. One of these layers was fertilized 

 with grape sugar, the other remained sterile. The spores were 

 sown sometimes in the sterile, sometimes in the fertile layer of 

 gelatine ; in the former case, the hyphge always grew (up or 

 down) into the sterile layer ; in the other case, they remained 

 in the fertile layer. If both gelatine layers were sterile, or if 

 both were equally fertile, the hyphse did not grow through the 

 holes in the membranes. Not the holes, but the stuff diffusing 

 from them, determined the direction of growth of the hyphse. 



There is a close relation between the chemical constitution 

 of the agents and their effects. The following substances 

 are attractive : compounds of ammonium (ammonium nitrate, 

 chloride, malate, tartrate), phosphates (of potassium, sodium, 

 ammonium), meat extract, peptone, sugar, asparagin, lecithin, 

 etc. The following are neutral : glycerine and gum arable (1 

 to 2%). The following are repellent : all free inorganic as 

 well as organic acids, alkalis, alcohol, and certain salts, e.g. 

 potassium-sodium tartrate, potassium nitrate, calcium nitrate, 

 potassium chloride (2%), potassium chlorate (8%), magne- 

 siurii sulphate, sodium chloride (2%), ferric chloride (0.1%), 

 phosphoric acid, etc. Comparing this list with that which 

 Pfepfee and Stange tried on swarm-spores (Pt. I, pp. BB- 

 SS), we find that there is a rather close correlation. In both 

 cases, glycerine (a good food) is neutral, alcohol repellent, 

 phosphates attractive. As in the reaction of swarm-spores, so 

 in those of hyphee, Weber's law is followed. 



5. Chemotropism of Conjugation Tubes in Spirogyra. — 

 This case is closely allied to chemotropisni of pollen-tubes. 

 Attention was first called to it by Overton ('88), who 

 observed that at the point where the tubes were about to arise 

 bacteria accumulated from the surrounding water. From this 

 phenomenon, and on other grounds, he was led to conclude 

 that a substance is excreted at this point which exercises a 

 directive influence upon the conjugation tubes, insuring their 

 meeting. This conclusion has been confirmed by Habeeland 



