In his publication about an epidemic of virescences in the years 1893 and 1894, Hugo de Vries pointed out that some virescences are caused by parasites, whilst others must be caused by other agencies and there are many more cases about which we are still in the dark. Some can be multiplied in an unlimited quantity by cutting them, e.g. the wellknown vegetatively constant „green rose” (Rosa indica) and the green Pelargonium zonale where the flowers have turned into rosettes of green leaves. In these forms we occasionally come across a reversion to the normal type. Whereas these virescences are most probably hereditary, we should regard the abnormalities, caused by parasites as non-hereditary, as modifications. De Vries examined many virescences a.o. in Crepis biennis, Agrostemma Githago, Aster tripolium, Dipsacus sylvestris and Lysimachia vulgaris. It was only in the case of Lysimachia vulgaris that de Vries succeeded in finding Phytoptus as the probable contaminator. The epidemic of virescences, however, left the impression that there was no doubt of a contagion. Moreover, sometime before, Peyritsch had succeeded in proving the contagion by artificial infection by means of Phytopts. Consequently it can be easily understood that in many cases where no definite cause could be assigned, Phytopts were looked upon as the originators of the virescences.