36

Scientific Memoirs by

is impossible to explain the grouping except as the result of invasion of an area
close to the site of discharge of the zoospores.

     The phenomena which have just been described are characteristic of those
periods of the year in which the alga is in full activity, during the rains and
beginning of the cold weather. As the cold weather advances, however, more
and more cells tend to assume a resting condition, and ultimately all persisting
ones doing so, the alga disappears for the season from the leaves of the host-
plant, the old leaves containing the resting cells gradually dying off and there
being no source of infection for those which replace them. The passage of the
cells from the active to the resting stage is attended by the following pheno-
mena. A very great accumulation of starch takes place, and at the same time
the colour of the cell-contents gradually changes from deep green to yellow, and
ultimately to deep reddish orange. The character of the cell-wall also changes.
It becomes greatly thickened and often very irregularly, so that it no longer
presents an even outline, but is provided with a series of irregular projecting
masses (Plate III, fig. 7). The contents appear to undergo a process of
condensation and shrink away from the cell-wall, and as they do so they become
invested by a separate bounding stratum of considerable thickness. In old
resting cells there is frequently a more or less pronounced brown colour due to
browning of the thick outer cell-wall. They vary very considerably in size, but
have an average measurement of 34 by 38 µ. The details of measurement
obtained from an isolated cell in the sediment deposited in a vessel containing
infected leaves were as follows: length 55 µ, breadth 36 µ, thickness of the coats
varying in different parts from 4.7 to 9 µ, diameters of the orange mass of
contents 43 by 31 µ. In certain cases the development of resting cells is
imperfect, and after a great accumulation of amyloid corpuscles has occurred
there is no development of yellow or orange colouring matter, but a mere process
of fading and disappearance of the chlorophyll, so that the cell is ultimately
reduced to a sac containing a colourless mass of starch grains. After the
resting cells have once been fully developed they remain unaltered for months,
and as the leaves containing them die off and decay, they become free in the
sediment of decomposing host tissues, During the cold-weather months they
remain unaltered, and on the onset of the hot season only do they begin to
change. The colour gradually changes from orange to green, the starch grains
for the most part undergoing solution at the same time, and cells come thus,
save in respect of the thickness of their walls, to resemble ordinary zoosporangia.
A formation of zoospores next takes place, and on the emergence of these a fresh
succession of active generations recurs, until the approach of the cold weather
heralds in a gradually increasing production of resting cells.

     The alga, whose life history has just been described in certain points, very
closely resembles Chlorochytrium, as originally described by Cohn¹and subs-

¹Beiträge zur Biologie der Pflanzen. Zweites Heft. 1872, S. 87.