12
THE ESSEX NATURALIST.
ground becomes sufficiently wet. It should interest members
of the Essex Field Club to know that in the Annals of Botany,
for 1917, Miss A. Piercy has published a careful study of the
structure and mode of life of a form of this species which occurred
on bare stretches of ground on a piece of grassland at Woodford.
Another alga which is sometimes responsible for patches of
green on the ground, although it more often occurs on the lower
parts of old brick and stone walls, etc., is Prasiola crispa, very
frequently in its entirely filamentous form, sometimes referred
to as P. parietina. These plants also are related to Ulothrix,
although placed in a separate family. They are remarkable for
the way in which filaments of single cells proceed under certain
conditions to produce flat expansions of from two to many rows
of cells, while the said expansions commonly reproduce by
detaching single cells or a row of single cells from their margins'
Generally speaking it appears that the forms remaining entirely
filamentous, with only one or occasionally two rows of cells, occupy
the drier positions, whereas the more expanded forms with num-
erous rows of cells favour more constantly wet conditions.
Besides the foregoing, green patches on the ground may be
caused occasionally by species of Chlorococcum, Vaucheria, Zygo-
gonium and many other algae, but it would be impracticable
on this occasion to enumerate all the possible producers of these
appearances. And it is much the same with the green coloration
on stones and rocks. All the species so far mentioned may be found
at times in such situations, and in addition certain species of
Desmids may occur in such abundance on rock and cliff faces that
they give a noticeable green effect to them. My friend, Mr.
G. T. Harris, tells me that he has seen Cosmarium larvae and
Mesotaenium macrococcum growing in this way.
Red and orange-red patches may be caused by one or other of
several species of alga. Perhaps the best known case is that of
Trentepohlia aurea, a filamentous alga of very specialised type,
which forms expanded sheets of bright red or orange-red colour
on the vertical faces of rocks in the hilly and mountainous districts
of the west and north of England, e.g., Dartmoor, Pennines, etc,
But the most striking example of red coloration on the ground
that has come to my own notice was observed in the lane just
inside the edge of Epping Forest between Warren Hill and
Loughton. Last year a certain amount of clearing was effected