272 DISEASES OF FIELD & GARDEN CROPS. [CH. 



It frequently happens that young lettuces grown in 

 frames in the spring are badly attacked, and in many 

 instances wholly destroyed ; the equable warmth and 

 humidity of a plant -frame is highly favourable to the 

 growth of this fungus. When frame lettuces are attacked, 

 a good plan for the destruction of the fungus is to give as 

 much air as practicable, and if possible to leave the frames 

 open for at least a part of one cold night. A short exposure 

 to cold or slightly frosty air will not materially hurt the 

 young lettuces, but will so cripple the vitality of the 

 fungus that the lettuces, on being planted out, will often 

 be found quite free from the parasite. With both lettuces 

 and onions, however, it has been observed that trans- 

 planted examples are often the weakest, and these weakest 

 plants are worst affected by their respective mildews. 



The existence of Peronospora ganglioniformis, Berk., is 

 preserved through the winter by the means of oospores or 

 resting-spores. The resting-spores may sometimes be easily 

 found in the old rotting stems of lettuce plants which have 

 been destroyed by the fungus. They are generally most 

 abundant between the spiral vessels and the external 

 shells of old lettuce stems. A group of the resting- 

 spores, as found by the Rev. J. E. Vize, near Welshpool, 

 is illustrated at Fig. 126, enlarged 400 diameters. The 

 oospores grow in enormous conglomerated lustrous 

 masses : so profuse is this, their natural habit of growth, 

 that under the microscope the masses of oospores look 

 like the shining roe of some fish. As in the fungus of 

 the potato disease, the resting-spores not uncommonly 

 grow within the spiral vessels as illustrated. These 

 oospores hibernate in rotting lettuce refuse during the 

 winter, and germinate in the early spring. On germina- 

 tion the first produced conidia perish, unless they alight 

 on lettuces or other suitable plants. When garden lettuces 

 are not near, the fungus is nursed by several common 

 cruciferous weeds; of these the worst is said to be the 

 common groundsel, Senecio vulgaris, L. The statement is 



