xv.] MILDEW OF TURNIPS AND CABBAGES. 85 



The resting-spores of this fungus have also been detected 

 in the wallflower, Cheiranthus Cheiri, L. ; the Shepherd's- 

 purse, Oapsella Bursa-pastoris, D.C. ; Gamelina sativa, 

 Cranz., and in other plants. 



Peronospora parasitica, Pers., not only grows on the 

 wild and all the cultivated varieties of the cabbage and 

 turnip, Brassica oleracea, L., and B. campestris, L. ; but it 

 often grows on Whitlow Grass, Draba verna, L., and on 

 the Shepherd's -purse, Gapsella Bursa-pastoris, D.C., in 

 company with one of the white-rust fungi named Cystopus 

 candidus, Lev., described farther on. It also grows on 

 Garlic Mustard, Alliaria officinalis, D.C. ; Pennycress, 

 Thlaspi arvense, L. ; Tower mustard, Ardbis perfoliata, L. ; 

 Coral root, Dentaria bulbifera, L. ; D. heptaphyllos, Clus. ; 

 Neslia paniculata, Des. ; Hairy Bittercress, Cardamine 

 hirsuta, L. ; Narrow-leaved Bittercress, G. impatiens, L., 

 and other plants. 



The many common weeds just mentioned act as nurse 

 plants for the putrefactive mildew of our turnips and 

 cabbages. The fungus lives through the winter in a 

 hibernating state not only in rotten turnip and mangel 

 roots, but in the decaying remains of such extremely 

 common weeds as the Shepherd's-purse and other worth- 

 less plants. It is obvious, then, that it is not only desirable 

 to burn all fungus-infected turnip and cabbage material, 

 but as far as possible to keep the fields and hedgerows 

 clear from the cruciferous weeds just mentioned. It 

 may be answered that it is impossible to keep down the 

 weeds and burn the decaying cruciferous rubbish. This 

 may be partially true, but the moral to be drawn from 

 the life history of this fungus is, Do not let putrid refuse 

 and worthless and dangerous weeds interfere more than 

 is necessary with the healthy growth of food-plants. 



