302 Trow . — Observations on the Biology and 
ultimum the cell-wall does not readily give the cellulose 
reaction with iodine and sulphuric acid. The reason for the 
difference has still to be accurately determined. The sum of 
the characters in Pythium , which is of course the best test 
of affinity, compels us to assign it a place in the Peronospora- 
ceae. In this group it occupies, as is generally admitted, the 
lowest place. The adherence to the aquatic habit, the fre- 
quency of the saprophytic mode of life, the rudimentary type 
of oospore wall, and the crude nature of its attack on its host — 
if parasitic, destroying rather than enslaving— serve to dis- 
tinguish it sharply from the more highly placed genera such 
as Peronospora and Cystopus. It would be interesting to have 
a more detailed knowledge of Phytophthora. In the present 
state of our knowledge it seems to be more closely related to 
Pythium than to the other two genera, and to occupy roughly 
an intermediate position. 
General Considerations. 
It had been hoped that incidentally this research would have 
thrown some light on that difficult and engrossing problem — 
the alternation of generations in plants. I (’99) have already 
indicated in the Annals of Botany how a study of the cytology 
of the Thallophytes may be expected to throw light on this 
question. It may be well to call to mind that plants are 
known to possess two kinds of gametes, those as in Fucus , where 
the nuclei possess half the usual number of chromosomes ; and 
those, as in Lilium , where the nuclei possess the full number. 
When the reduction in the number of chromosomes takes 
place in oogenesis and spermatogenesis, there cannot well be 
an antithetic alternation of generations. However great the 
diversity in the different generations due to special adapta- 
tion to the environment, we never get further than poly- 
morphism of the gametophyte. When antithetic alternation 
typically occurs the reduction is known to take place during 
the development of the spores. The fact that plants certainly 
present these two definite cases is not even yet properly ap- 
preciated. 
