Observations on the Biology and Cytology 
of Pythium ultimum, n. sp. 
BY 
A. H. TROW, D.Sc, F.L.S, 
With Plates XV and XVI. 
OR some years a detailed knowledge of the cytology 
X of the genus Pythium has been a great desideratum. 
Notwithstanding the attention which has been bestowed on 
the Phycomycetes, and especially on the Oomycetes in recent 
years, and the great interest which has been excited by the 
results achieved, this very interesting genus has been prac- 
tically neglected. We need only to refer to the work of 
Wager (’89, ’96, and ’00) on Peronospora and Cystopus ; 
Berlese (’97) on the same and other genera ; Stevens (’99) and 
Davis (’00) on Albugo ( Cystopus ) ; Istvanffi (’95) on Cystopus , 
Saprolegnia , and other genera ; Humphrey (’92), Hartog (’95), 
Dangeard (’90), and myself (’95, ’99) on Saprolegniaceae 
(chiefly Saprolegnia and Achlya ) ; and Lagerheim (’00) on 
M onoblepharis , to show how rapidly our knowledge of the 
cytology of the Oomycetes has grown in the last decade 
of the nineteenth century. Little, however, has been done 
for the genus Pythium since the time of Pringsheim and 
De Bary (’87), to whose fundamental work it is no longer 
necessary for us to refer at length. Ward (’83) alone in this 
country seems to have paid attention to the genus, and his 
[Annals of Botany, Vol. XV. No. LVIII, June, 1901.] 
