xxxiv.] LETTUCE MILDEW. 271 



glandular hairs of a sundew leaf, or like a snail's tentacle 

 with its eye-bulb. P. parasitica, Pers., has no saucer-like 

 expansions. One of the exquisitely fine tentacle-fringed 

 saucers with a spore, balanced on one tentacle, is enlarged 

 to 1000 diameters at A. At the top of the spore at B, 

 a small, almost invisible, papilla may be seen. This is the 

 weakest point of the spore, and at the time of germina- 

 tion this part dissolves away or bursts, and the contained 

 material pours out as a germinal thread. The spawn 

 threads of this species grow within and upon the leaf, 

 and attach themselves to the constituent cells in a manner 

 similar with the cabbage Peronospoar, illustrated at Fig. 29. 

 In whatever part of the leaf or stem the spawn grows it 

 there sets up putrescence. The mycelium within the leaf 

 is thick and coarse, and much too stout for emergence 

 through the organs of transpiration. To meet this diffi- 

 culty the spawn becomes naturally flattened as it ap- 

 proaches the stomata, and it emerges through the little 

 slit on the under surface of the leaf, with a chisel edge, 

 as does P. parasitica, Pers., see Fig. 30. As soon as the 

 spawn thread of P. ganglioniformis, Berk., reaches the air 

 it commonly branches, and, instead of a single fruiting- 

 branch depending from each stoma there may usually be 

 seen a small bunch of threads or conidiophores. 



This parasite causes pallid patches of decomposition to 

 appear on lettuce leaves, and on the margin of these 

 patches little white knots of the destructive mould may 

 be readily seen with the unaided eye. In the spring the 

 pest generally shows itself first on the outside leaves near 

 the ground where the air is still and humid, and grows 

 inwards to the heart of the lettuce, carrying decay in its 

 course. In bad cases summer lettuces are quickly reduced 

 to putrescent masses. In the autumn the flowers and 

 seeds are chiefly damaged ; sometimes the harvest of seeds 

 is totally destroyed. Thickly sown plants are worst 

 affected. The fungus lives on the lettuces all the season, 

 from spring to October. 



