26 THE ESSEX NATURALIST
Fig. 4. Distribution of Oak and Hornbeam.
grow right down to the streams, or, in the absence of the oak,
the hornbeam would grow much further up the sides of the
valleys; where, however, the two are competing, moisture
determines the extent of spread of each species.
Fig. 5 shows the distribution of the commonest species of
the field layer, the bracken fern (Pteridium aquilinum).
Three definite zones can be distinguished — regions where
bracken is dominant, sparse and absent. Comparison of this