FRESH-WATER ALG. 247 
A remarkable plant allied to it, called Thorea 
vamosissima, occurs in the Seine and in several 
other rivers on the continent, and has been found 
in this country in the Thames. It is attached dur- 
ing Juneand July to the bottom of boats, to stones 
and weeds. The whole plant, which is often many 
feet in length, is covered with a fine mucous down. 
Its colour is dark green when growing in the water ; 
but it assumes a scarlet-tint when pressed on paper, 
staining pieces of cotton with it, and it settles down 
into a fine dark purple colour when perfectly dry. 
On shady walls and thatched roofs, at the foot 
of rocks and houses in damp situations, may often 
be seen a stratum of densely crowded transparent 
green leaves, plaited and wrinkled with rounded 
lobes. This plant, called Ulva crispa, is the ter- 
restrial variety of the familiar green laver of the 
sea-coast. Another species of the same genus 
(U. bulbosa), occasionally fills stagnant pools and 
ditches of fresh water, with its excessively soft and 
lubricous masses, appearing as if in a state of fer- 
mentation. It is exactly the counterpart of the 
common sea species. The Exteromorpha intesti- 
nalts, with which every visitor to the sea-coast is 
familiar, adding greatly to the beauty of rocky 
pools, left full by the receding tide, also occurs 
not unfrequently in fresh-water ponds and stag- 
nant waters in spring and summer. An allied 
