On British shores Pylaiella littoralis occurs in a large number of local populations which usually fall into one or other of four population groups. These populations often differ in habit, form, seasonal distribution, etc.

Two populations of P. littoralis from different environments have been studied experimentally and it seems from this that the morphological differences between the plants from the two populations are accompanied by physiological and genetical differences. Similar results have been obtained from experiments on other populations.

It is suggested that the numerous populations arise through the selective effect of all the prevailing environmental factors and are adapted to them.

The potentially large number of plant forms resulting from this process should not be described in terms of ‘orthodox’ taxonomy though they may possibly merit definition in terms of ‘experimental’ taxonomy in so far as some of the populations may constitute ecotypes as recognized in higher plants. Because it is possible for a single ecotype to be represented by more than one morphological type, however, the value of the ‘experimental’ system is regarded as doubtful.