20 
THE SEAWEEDS 
Order CONFERVOIDEAE. 
Family ULVACEAE. 
These are among the first seaweeds that meet the eye of the naturalist 
on reaching the shore, for their green colour renders them conspicuous as 
they grow on the sides of the rocks or in the rock pools. The fronds are 
quite simple in form and in structure, consisting of a membrane composed 
of uninucleate cells, each a small green mass surrounded by a colourless 
margin. The thallus is formed of one layer or, more usually, of two layers; 
the chlorophore of each cell is continuous. The frond is foliose or hollow; 
the leafy fronds are of a single layer ( Mono stroma ) or of two layers 
{Viva). The hollow or tubular fronds are entitled Enteromorpha. 
ULVA Linnaeus. Sea Lettuce or Green Laver. 
The Sea Lettuce shows great variation in the form and the colour of 
the fronds. On rocks exposed at low tide it is often ragged and shapeless 
and usually pale from the action of sunlight. In deeper and less disturbed 
water it assumes more definite shapes; the fronds are larger, ovate or 
rounded, and of a deeper green. The young plants consist of a small 
rounded frond attached by a short cuneate stipes which broadens out above 
into the frond. Later the stipes may become inconspicuous and the plant 
appears to be nearly sessile, or the whole basal part may become thickened,, 
an intermediate layer forming filamentous processes which become 
irregularly interwoven and contribute to the fixing of the plant to the 
substratum. The frond does not grow quite in one plane and the margins 
are in consequence waved and furbelowed. They are often perforated by 
larger and smaller holes due to the molluscs which feed on them, and often 
bear other small algae as epiphytes. 
There are no conspicuous organs of reproduction. Quite normal looking 
cells are found at times with the contents breaking up into a number of 
small rounded bodies, which are ciliated and free swimming. These are 
of two kinds, which may or may not be produced by the same individuals. 
The first are smaller, with two cilia, and are quite similar sexual gametes; 
the union of pairs of these form zygotes; the zygote develops directly into 
a new thallus. The other bodies are larger, with four cilia at the anterior 
end; they are non-sexual zoospores. They become attached to a substratum, 
lose their cilia, and grow out into a filament, out of which, by gradual 
subdivision of the cells, a normal frond develops. 
It is not a simple matter to observe the release of the spores from the 
cells and the union of the gametes. Dr. C. L. Anderson described his 
observations to the Microscopical Society of San Francisco. He gathered 
