E. J. BUTLER. $3 
sieve tubes with their companion cells. In diseased leaves, the 
vessels are increased in size and number, groups of two or three 
large vessels occurring on either side (plate IV, fig. 4). The sieve 
tubes are increased in number and are slightly larger than normal. 
These modifications occur both in the sheath and blade of attacked 
leaves. From the collapse of the central layers of parenchyma 
which follows on sporangium-formation, and the consequent 
flattening of the leaf, the bundles are, in old cases, often distorted, 
being broadened transversely or sometimes infolded, so that the 
xylem projects laterally outside the phloem. In these cases the 
large vessels are flattened to a remarkable degree (plate IV, fig. 5). 
Sporangia are produced only on the normal leaves of the host, 
never in the inflorescence. Following on the appearance of the 
whitish streaks which are the first outward sign of the presence of 
the parasite in the leaves, a cloud of thick, pale conidiophores bursts 
from the stomata, covering the surface of the streaks with a greyish 
white haze easily visible to the naked eye. A single or several 
conidiophores may project from each stoma (plate V, figs. 5, 6). 
They are broad, rather short, stalks, measuring about 100, in 
length, by 12-154, unbranchedin the lower part, but usually 
with a few short, thick branches, dichotomously or trichotomously 
formed at the top, and crowned with numerous papille of charac- 
teristic shape, on which the sporangia are borne. ‘The latter are 
hyaline, broadly elliptical, slightly pointed at the free end, with 
a thin smooth wall, and 22-30# by 12-16 in diameter. They 
fall early without any stalk, and germinate rapidly in water, liber- 
ating zoospores (plate V, fig. 8). The number of zoospores varies 
with the size of the sporangium, from three or four up to a dozen 
or more. They are irregularly kidney-shaped, unequal sided, flat- 
tened bodies, with two cilia from the concave side, the posterior of 
which is the longer (plate V, fig. 8). The zoospores after coming 
to rest round off, measuring 9 to 12” in diameter in the quiescent 
state, and germinate rapidly by a hypha. 
After the formation of sporangia the conidiophores early col. 
lapse, the whole asexual stage of reproduction being evanescent, 
and only to be found in young plants. Once the characteristic 
