13 



in which such a change takes place in nutrition. The experiments under- 

 taken at Pfeffer's Institute in Leipsic show that fungus hyphae are irritable 

 chemically and that the direction of their growth may be influenced either 

 towards the stimulating substance, (positive chcmotropism) or away from 

 it (negative cJiemotropism). Indeed their mode of growth also can be 

 changed since, for example, a tendency towards sprout formation sets in with 

 a higher concentration of the solution. The commonest mold species, which 

 occasionally become parasitic (Mucor, Penicillium, Aspergillus) show an irri- 

 tability with substances which almost always can be presupposed to be char- 

 acteristic of phanerogamic plants. Besides dextrin and the neutral phosphoric 

 acid salts, sugar especially attracts fungi, in case the concentration is not 

 too high. Thus, for example, grape sugar in a 50 per cent, solution acts repel- 

 lently for Mucor stolonifer, the active agent of decay of fruits. .A.cids, on the 

 contrary, and alkalis from the beginning act repellingly. The germination 

 tubes of the summer spores of Uredo linearis, a grain rust, are attracted by a 

 decoction of plum and wheat leaves. Especially interesting are the cultural 

 results with Penicillinni glaucum, whose hyphae bore through the cell walls 

 of a leaf impregnated with a 2 per cent, cane sugar solution. In the same way 

 they penetrated artificial cellulose membranes and the epidermis of bulb scales 

 which lay on a nutrient gelatine. 



These are especially important clues capable of explaining the numerous 

 case of sickness from Penicillium. It is well known that this mold, the 

 most abundant agent of decay in stone fruits, first begins to spread when the 

 ripening process has converted the starch into sugar. 



In connection with the penetration of Penicillium into the scales of 

 bulbs, we find abundant examples in the cases of decay in the tulip, hyacinth 

 and lily bulbs which occasionally lead to lawsuits. This decay occurs especially 

 extensively when wet years prevent the maturing of the bulbs or if the bulbs 

 are stored when containing an unusual amount of sugar and then used pre- 

 maturely for forcing. 



Thus zve see hozv the cell contents and the cell ivalls of the host plant 

 can determine the penetration of hyphae and the transition of the saprophyte 

 into a parasite. 



4. Parasitic Diseases. 



Supported by various carefully studied cases of parasitism, many ob- 

 servers so generalized the conception of parasitic diseases that they assumed 

 them to be present wherever organisms are found gathered together. In 

 many cases this is supported by experiments in which the parasitically living 

 organisms were injected into the host and were able to produce a local dis- 

 ease in the tissue. 



With this method the apparent proofs of parasitic disease were accumu- 

 lated in such a way that one was forced to the assumption that there could 

 be scarcely any disease which was not caused parasitically- Infection ex- 



