70 



NA TURE 



\May 25, 1876 



THE POTATO DISEASE^ 

 II. 



THE Peronosporea: are usually divided into two genera, 

 viz. — Cystopus and Peronospora : with the former 

 the potato disease has no connection. 



Fig. t.— Peronospora infestans, Mont., x 250 dia. From De Bary's slide. No, III. a, a, septate 

 conidiophoros ; B, B, vesicular swellings ; c, c, immature conidia ; d, conidium. showing differen- 

 tiation of its contents ; e, e, free zoospores ; f, frequent mode of attachment of branch. 



II.— Peronospora, Corda. 



The fungus of the potato disease was first placed under 

 the genus Botrytis Mich., but Corda established Perono- 



I Continued from vol. xiii. , p. 527. 



spora to receive the species with non-septate threads. 

 Unfortunately for the genus, and for P. mfestans in par- 

 ticular, the threads of the latter are always more or less 

 septate, and this character effectually separates the 

 potato-fungus from the Saprolegttiece (as at present de- 

 fined) where septa are unknown. 

 De Bary now proposes the estab- 

 lishment of another new genus 

 (under the name of Phytophthora) 

 to receive the potato-fungus, the 

 chief character of the proposed new 

 genus resting on the development 

 of, not one, but several spores (co- 

 nidia) successively at the end of 

 each branch of the aerial threads 

 of the fungus (conidiophores). This 

 character has been known to fun- 

 gologists since the potato-fungus 

 was first described, although it has 

 generally been esteemed of specific 

 rather than of generic value. The 

 Rev. M. J. Berkeley in illustrating 

 the potato-fungus for the Royal 

 Horticultural Society, figures the 

 conidia as being pushed off the 

 branches (PI. 13. Fig. 12*, 14). The 

 same phenomenon is illustrated in 

 the Micrographic Dictionary (Pi. 

 20, Fig. 6). I have also recorded 

 the habit in the secondary condition 

 of the fungus where the oogonia are 

 successively pushed off the support- 

 ing threads. 



As so much attention has of late 

 been directed towards the Perono- 

 sporeas it is more than ever neces- 

 sary that the characters of the 

 family should be correctly known, 

 and this is especially important as 

 regards the nature of the secondary 

 state of these fungi : a brief state- 

 ment of the sexual condition may 

 therefore be useful. The female 

 cells (oogonia) are borne sometimes 

 on the tips of mycelial threads, 

 sometimes as sessile bodies on dif- 

 ferent parts of the threads and 

 sometimes intercalated within the 

 threads themselves, after the man- 

 ner of the illustrations on Figs, i, 

 3, and 4. The male organs (an- 

 theridia) are usually smaller and 

 carried on finer threads, not as a 

 rule anatomatically distinct from 

 the oogonium - bearing threads. 

 The contact of the antheridium 

 with the oogonium, gives rise to 

 the oospore or resting-spore. 



Peronospora infestans, Mont., in 

 its aerial state has been so often 

 and so accurately described by 

 Berkeley, De Bary, and others, that 

 any further illustration or descrip- 

 tion is almost unnecessary. It 

 must be confessed, however, that 

 the figures first published by Messrs. 

 Berkeley and Broome (in connec- 

 tion with Dr. Montagne) and lat- 

 terly by De Bary, have only too 

 many times been copied and re- 

 copied without reference to the fungus itself. There is 

 therefore no apology required for publishing the present 

 illustration (P'ig. 7) which is new, and I trust exact. It is a 

 camera lucida reproduction enlarged 250 diameters of a 

 group of conidiophores as supplied by Prof. A. De Bary 



