156 illCEOSCOPIC FUNGI. 



oiiion_, but appears on otlier allied species of AUlinii 

 (to wliicli the onion belongs). The threads form 

 large patches or blotches on the leaves, and some- 

 times cover them entirely. It very much resembles 

 the turnip mould, from which the form of the spores 

 considerably differs. This is the same species as 

 that described by Caspary, and afterwards by 

 Berkele}', under the name of P. destructor. 



Lettuce Mould. — A very similar mould {Ptrono^ 

 spora gangJiformis) is sometimes very common in 

 spring on the under surface of the leaves of the cul- 

 tivated lettuce, appearing in definite white mouldy 

 spots. By reference to the figure of a portion of a 

 thread magnified (fig. 265), it will be seen that the 

 peculiar form of the tips of the branchlets evidences 

 the distinctness of this species. 



The oospores are small, globose, and of a yellowish 

 tawnv colour. This mould is bv no means confined 

 to lettuces, but has also been found on species of 

 ragwort, sow-thistle, nipplewort, endive, and other 

 composite plants ; and has from time to time 

 received numerous names, which it is unnecessary 

 to enumerate. 



Taee Mould. — The under surface of the leaves of 

 tares, and sometimes also of peas, is liable to at- 

 tack from an allied species of mould [Peronospora 

 Vicice). In the spring of 1846 it appeared amongst 

 vetches in some districts to such an extent as at 

 one time to threaten the destruction of the crops ; 

 but a succession of dry weather at once abridged 



