ON THE PERONOSPORiE. 199 



slightly dilated at the tip, and on each tip a comparatively large 

 globose spore is balanced. The mycelium within the leaf is fur- 

 nished with minute suckers, with which the fungus pierces the cells 

 and supports its life. The mycelial threads are too large to get 

 through the minute stomata of the leaf. To meet this difficulty, 

 the threads inside the leaf, as they approach the stomata, twist 

 round once like a corkscrew, flatten themselves, and push through 

 the stomata with a chisel edge. As soon as this chisel edge is 

 pushed into the air, a septum appears, and from this septum one, 

 two, or three fruiting-threads arise. 



Happily, the Lettuce Peronospora is not everywhere in Britain. 

 It is recorded as growing on sow-thistles in Scotland. Some 

 market-gardeners have never seen it ; others know its spring and 

 autumn visitations too well. It causes large pallid patches to 

 appear on the leaves, and on the outside of these patches, little 

 white nodules of the destructive mould may be seen. In the 

 spring the pest begins on the outside leaves, and proceeds inwards, 

 carrying putrefaction in its course. In bad cases, summer lettuces 

 are quickly reduced to lumps of wet putridity. This fungus grows 

 on other Composite plants^, both wild and cultivated, such as 

 endive, groundsel, nipplewort, and sow-thistles. Weeds nurse the 

 fungus for the lettuces, and as P. ganglioniformis is known to pro- 

 duce resting-spores, it is desirable that all decaying lettuce mate- 

 rial, as well as the decaying weeds above mentioned, should when 

 possible be gathered and burnt. 



P. Parasitica. Cabbage Peronospora. 



Threads of the mycelium thickened and much branched ; 

 suckers numerous and branched ; branches clavate, obtuse ; fer- 

 tile threads thick, soft, flexile, equal or unequal ; five to eight times 

 dichotomous, rarely trichotomous, branches always repeatedly tri- 

 furcate; acrospores broadly elliptical, very obtuse at the apex, 

 white. This fungus invades cabbages, turnips, and other crucifer- 

 ous plants, as shepherd's purse. Cauliflowers are sometimes at- 

 tacked by this fungus. Yellow spots appear on the upper surface 

 of the lower leaves, which, when examined on the lower surface, 

 are found glaucous with the mould, which usually soon destroys 

 the whole plant. In turnips the leaves seem to be first attacked ; 

 then the root itself becomes covered with waved, irregular lines^ 



