418 BULLETIN OF THE BUSSEY INSTITUTION. 
by slender necks. A magnifying power of three hundred diameters is 
sufficient to show them plainly. In the leaves, the hyphze are found 
in all parts except the vascular bundles, and are more irregular in 
diameter than in the stem, being often swollen in a varicose manner. 
Just beneath the stomata, the hyphz are particularly abundant and 
intricately entangled. Those which are destined to bear the conidia 
pass through the stomata being constricted in their passage and 
expanding afterwards, so that when cut off from the surface of the 
leaf they seem to have a bulbous base. Sometimes such a large 
number of hyphz force their way through a single stoma that the two 
cells which bound it are torn asunder. 
However large a number of hyphz may force their way through, a 
small number, from four to eight, grow faster than the rest and bear 
the conidia. The conidia-bearing hyphe vary from .2 to .6 mm., in 
height when fully matured. Their general habit of branching is 
shown in Plate II., Fig. 1. The tip of the simple axis divides into 
three parts, one of which generally seems like the direct termination 
of the axis and the other two as lateral offshoots, given off at about the 
same level. At the base of each of the two lateral offshoots, two 
similar secondary offshoots are again given off as shown in Plate IL., 
Fig. 2, which represents a highly magnified view of a tip of a stem 
in which @ denotes the primary divisions and 6 the secondary. The 
third division of the primary branch on the left is behind, and con- 
cealed from the observer by the branch itself. The same form of 
division takes place at the tip of the branches and their subdivisions, so 
that the ultimate ramifications always seem three-parted. The branches, 
which are few in number, generally from four to eight, are placed 
alternately on the upper third of the axis, being generally, but not 
always, distichously arranged. Relatively to the main axis, they are 
all short, the broadest expansion from side to side not being usually 
greater than .12mm. ‘The branches are furnished with branchlets of a 
second and third order as shown in Plate IL, Fig 1, where the naked 
lower portion of the stalk has been only partly represented. The 
general arrangement of the conidia-bearing hyphe, it will be noticed, 
is more compact than in most species of Peronospora, the ultimate 
branchlets being very dense. The conidia are borne! in profusion on 
the tips of the hyphe. They are of an oval shape, obtuse, and with- 
out the papilla found in some species at the remote end, but slightly 
