skipper's island papers 333
covered by the highest spring tides. The lichens here are best
developed on the sides facing the open marsh (i.e. the S.-W. side).
That a zonation of species exists is apparent on inspection, but
this has been demonstrated quantitatively by means of con-
tinuous series of 2" square quadrats running from 2" to 20" above
soil level on ten selected posts 20" high. The presence or absence
of species was recorded in the individual squares and the results
are recorded in the histograms in fig. 1.
Figure 1. Histograms representing the distribution of the main lichens
ou the S.W. side of posts situated at about mean high water
mark. Ten posts 20 in. high were selected and on each was drawn
a continuous series of 2 in. squares running from 2 in. to 20 in.
The presence or absence of species was recorded in each square.
The vertical columns of histograms represent the number of
occurrences (out of the possible 10).