A NEW GENUS OF PARASITIC ALG. — 309 
sufficient, these soon commence to swarm actively, rupture the thin wall of the oospore, 
and, escaping in succession through the ostiolum of the oogonium, swim off freely into 
the subepidermal space and over the surface of the leaf. In size, colour, and general 
` appearance, these zoospores are precisely similar to those developed in the asexual fruc- 
tification. Like them, they are provided with two elongated flagella, one of which, so 
long as the body retains its original oval form and active motion, is usually trailed behind, 
whilst the other is employed in progression. Occasionally they become adherent to one 
another, or to other solid bodies, by their trailing filaments. Sometimes a pair of them 
may be seen to become adherent to one another by the extremities of these filaments, 
and, continuing their rapid rotating movement, to twist them so tightly up as to bring 
their bodies into close contact with one another. Under such circumstances, an actual 
fusion or conjugation appears in some cases to occur, resulting in the formation of one 
large biflagellate zoospore. Such a phenomenon, however, is very rare, and seemingly 
accidental. 
The zoospores, when first emitted, are of an oval or pear-shaped form, and average 
about 0:00825 x 0°0052 millim. in size. As they become less active and begin to change 
their form, they frequently increase in size considerably. Eventually they cease to 
move and become spherical, the flagella continuing to vibrate for some time, and, after 
becoming motionless, persisting for a considerable period as two delicate hair-like fila- 
ments, attached to a portion of the cell, which, by its comparative freedom from colour- 
ing matters, indicates the site of the original hyaline extremity. When examined under 
a high power at this time, the cells sometimes appear to be invested by a delicate halo of 
gelatinous matter (Pl. XLII. fig. 16, a). They are provided with a very delicate mem- 
branous wall, and contain a fluid with numerous reddish granules, and a mass of green- 
ish colour investing from ten to twenty, or even more, oval particles of considerable size. 
When confined beneath a cover-glass, the greater number of the cells next undergo the 
following changes :—A swarming movement begins among the reddish granules of the 
cell-fluid ; this attains a great intensity, the cell-wall bursts, an escape of granules occurs, 
and the green mass or globule is partially protruded from the cell. The body now pre- 
sents an appearance similar to that shown in the figure (Pl. XLII. fig. 16, 6). After 
remaining for some time in this condition, the oval particles within the green mass begin 
in their turn to swarm, and, ultimately escaping from it one by one, swim off and are 
dispersed in the fluid, leaving the green mass adherent to the fragments of the original 
cell. At the time of exit the oval particles measure about 0:001 x 0:0005 millim. 
Under normal circumstances and favourable conditions, however, many of the zoo- 
spores proceed to give origin to new plants of the parasite. The various stages in deve- 
lopment may be traced by means of specimens which may be obtained in abundance 
from the surface of the leaves, near patches of the parasite containing mature oospores, 
and which have been exposed to sufficient moisture. On examining such portions 
of the leaves under a low power, or even, in some cases, with the naked eye, they may be 
seen to be sprinkled with minute yellowish or orange specks. These are readily detached 
from the surface, and consist of the spores and young plants derived from them. The 
spores, on ceasing to move, become spherical and increase slightly in size. The. con- 
SECOND SERIES.—BOTANY, VOL. I. 2x 
