CELLULAR STRUCTURE 37 
The group of alge to which belongs the 
green sea-lettuce, so common on some of our 
coasts, especially where the sea-water is con- 
taminated by sewage effluents, furnishes 
beautiful examples of the simpler stages in 
the evolution of a flattened leaf-like type of 
thallus. Ulva, the sea-lettuce in question 
(Fig. 3), is somewhat advanced, for it consists 
of cells which are arranged in two layers, but 
otherwise division occurs in the cells of each 
layer in such a way as to increase the area of 
the surface. The multiplication of cells is not 
very uniform over the whole surface, those 
nearer the margins dividing and growing 
faster than those nearer the middle line of 
the leaf-like plant. Thus the surface of the 
frond is thrown into folds and wrinkles as 
the necessary consequence of this unequal 
growth. But that there is some co-ordinating 
influence at work amongst the cells is shown 
by the fact that this wrinkling does not become 
excessive, and the plants assume a fairly defi- 
nite form which makes any given individual 
easy to recognise as belonging to this and no 
other species. The Ulva plants are securely 
anchored to stones and other supports by 
a special development of the cells near the 
base of the plant. These grow out into long 
filamentous strands, and adhere very closely 
to the surface of the rock. The specimens 
one often sees washed up after a storm are 
usually the upper parts of the plants, which 
have become torn off by the waves. 
