THE FOOD OF PLANTS. 101 



shall find that numerous fungal hyphoe traverse the green tissue 

 of the stem, disorganising the cell-contents. The mycelium fur- 

 ther produces at particular places numerous outwardly directed 

 branches, which perforate the epidermis of the haulm, and ab- 

 strict from their ends unicellular spores (uredospores). The 

 uredospores then finally give rise to the teleutospore layers, which 

 we shall not here investigate. 



The common potato disease, which is so highly epidemic, is due 

 to a fungus which belongs to the Peroiiosporeee, and is known as 

 Phytophthora infestans. It may be observed in summer on the 

 leaves, but also in winter on the tubers, of Solanum tuberosum. 

 If diseased potato tubers (which may be easily recognised by the 

 presence of brownish, somewhat sunken spots on the skin) are cut 

 up. and we leave the pieces for about two days under a bell-glass 

 in air saturated with aqueous vapour, the cut surfaces become 

 covered with a delicate white " mould." The mycelium of the 

 Phytophthora is present in the diseased tubers, to begin with ; it 

 occurs in large quantities between the cells, and lives at the ex- 

 pense of their substance. Under the conditions described, it sends 

 out gonidiophores to the exterior. These gonidiophores, as micro- 

 scopical examination teaches, are branched in their upper parts 

 arid form sporangia, which, however, in contact with water readily 

 fall off. If pieces of diseased potatoes are allowed to remain 

 under the bell-glass for some time, a rich fungus vegetation de- 

 velops on them, which, however, has nothing to do directly with 

 the Phytophthora. 



It is also very instructive to follow closely the course of de- 

 velopment in a Peziza, viz.*P. sclerotiorum. It is one of the Dis- 

 comycetes, and causes the sclerotium disease of rape for example. 

 I will describe my method of cultivating the fungus. A number 

 of sclerotia are laid on the surface of moist earth contained in a 

 flower pot. The pot is covered with a glass plate, and allowed to 

 stand in diffused light not far from a window, care being taken 

 that the soil does not become dry. After six to ten weeks the 

 small, stalked sporophores of the Peziza develop from the sclero- 

 tia. A carrot is now cut up, and after the pieces have been 

 scalded in hot water, we infect them, by means of a sterilised 

 needle, with ripe spores from the Peziza cups, put them in a 

 crystallising glass, and cover with a bell-glass. After a few days 

 the spores germinate. A luxuriantly vegetating mycelium rapidly 

 develops on the surface of the pieces of carrot, which destroys 



