CHAPTEE XXXIV. 



LETTUCE MILDEW. 



Peronospora ganglioniformis, Berk. 



THE putrefactive fungus of lettuces was detected by the 

 Rev. M. J. Berkeley, and described and illustrated by him 

 in vol. i. of the Royal Horticultural Society, 1846. The 

 specific named ganglioniformis refers to the resemblance of 

 the fruiting-threads of the fungus to the natural enlarge- 

 ments termed ganglions in the course of a nerve. 

 Tulasne thought this parasite was a mere variety of P. 

 parasitica, Pers., as found on cabbages and illustrated in 

 this work at Fig. 29 ; but a reference to our illustration 

 of P. ganglioniformis, Berk., enlarged at Fig. 125 to 400 

 diameters, as compared with P. parasitica, Pers., enlarged 

 in Fig. 29 to 200 diameters, will show how distinct the 

 two are from each other. Professor de Bary disapproved 

 of Mr. Berkeley's specific name ganglioniformis, and substi- 

 tuted gangliformis for it, considering the latter more 

 correct, but no alteration was required. Had it been 

 necessary, the word gangliiformis, as printed by Dr. Max 

 Cornu, would be most correct. In both P. ganglioni- 

 formis, Berk., and P. parasitica, Pers., the fruiting- 

 stems or conidiophores and branches are flattened, and as 

 these flattened stems and branches twist a little -in the 

 process of growth, they have a spurious appearance of 

 swelling in a ganglionic manner. Each ultimate branchlet 

 of P. ganglioniformis, Berk., is beautifully dilated into a 

 saucer-like expansion, with a single excessively-attenuated 

 spicule growing from the centre of the saucer, and with 

 from three to five similar minute spicules growing round 



