16
BRITISH
DESMIDIACEJE.
water, and after the superfluous liquid is allowed to drain
away, squeezed over the wide mouth of the tube or
bottle. The sediment that settles to the bottom of the
bottle usually contains Desmids, more or less numerous.
One may always depend upon finding some Desmids in
material sqiieezed from permanently submerged Sphag­
num.
Round the rigid leaves of Isoetes and Lobelia there is
often quite a thin gelatinous coating of a yelloAvishbrown colour. This can be removed from the water
by getting the leaves between the fingers, with the
hand palm upwards, and then gently drawing the
hand upwards through the water. This method of
collection requires much patience and some practice, as
it is exceedingly difficult to raise in the open hand,
light, flocculent, gelatinous material a distance of
about two feet through the water. Most of the finest
and purest material we have ever examined has been
collected in this way.
All the larger Algse should be carefully collected,
because it is amongst these that some of the most
interesting Desmids are found. Similarly, the home
of many characteristic Desmids is amongst the mosses
and filamentous Algas which occur on dripping rocks.
In the sheltered corners of some lakes there is often
a growth of Phragmites or Scirjms lacustris, and scrap­
ings of the older stems of these plants frequently yield
good results. A net of coarse muslin or a coarse
copper strainer will be found to be very useful for
passing amongst submerged plants. For the examina­
tion of large ponds and lakes the use of a boat is of
great service and often indispensable.
To collect the plankton-material from large lakes,
tow-nets are necessary. These nets are conical in
shape and constructed of miller's silk ; they are six or
eight inches wide at the opening, and fourteen to twenty
inches in length. The open end should be sewn on to
coarse sail-cloth, the latter being doubled and fastened
to a stout copper ring, and then three equidistant holes