738 Wisconsin Academy of Sciences , Arts, and Letters. 
mosomes. The nucleus is thus shown to be a polarized structure 
throughout the life history of the fungus, unipolar in the rest¬ 
ing condition and becoming bipolar in division. 
The process of spore formation in Phyllactinia is the same 
as in Erysiphe b and, as is so frequently found in the mildews, 
only two nuclei are enclosed in spores, the remaining six de¬ 
generating. 
OBSERVATIONS. 
The mildews offer especially favorable material for the 
study of nuclear fusions, nuclear divisions and the very pecu¬ 
liar process of free cell formation which characterizes the as¬ 
cus, and the following study was undertaken for the purpose 
of extending our knowledge of the group by an account of the 
development of the ascus and spore formation in the genus 
Microsphaera. I have not undertaken to work over the earlier 
stages in the formation of the perithecium, but have directed 
my attention mainly to the question of the persistence of the 
centers during the processes of nuclear fusion and nuclear divis¬ 
ion, and to the process of spore formation in the ascus. I have, 
however, observed incidentally certain stages in the develop¬ 
ment of the perithecium bearing on the account given by !Ne- 
ger of the structure of the mature ascocarp and its ecological 
adaptations, and shall describe these observations in connection 
with my account of the structure of the nuclei in the ascogo- 
nium and ascogenous hyphae from which are formed the nuclei 
that subsequently fuse in the young asci. 
As is everywhere commonly observed in this country, Micro¬ 
sphaera alni DC. grows in great, abundance on Syringa 
vulgaris —the common lilac—covering the leaves with a white 
cobwebby mycelium dotted with the dark specklike fruit bod¬ 
ies, and furnishes an abundance of material in all stages of de¬ 
velopment for cytological study. The fungus shows a radial 
growth, so that infected spots may have nearly all stages, from 
mature perithecia in the center to the youngest fruits on the 
periphery. 
Small squares of leaf covered with the mycelium and peri- 
thecia were fixed in Flemming’s stronger solution of ehrom- 
osmic-aeetic acid, Flemming’s weaker solution, and Flem- 
