184 Rhodora [OCTOBER 
about natural succession under the prevailing climatic and edaphic 
conditions, and will furnish data on the means of dispersal of species 
in this region. | 
ECOLOGY OF PENIKESE.! 
The ecological survey of the island is here divided into three parts: 
beaches, grassland, and ponds. 
Beacues. The island is entirely surrounded by a beach of varying 
width and character. There is a plantless zone between the marine 
flora consisting almost entirely of algae, and the true beach flora of 
seed plants. This plantless zone is determined by the tides, being 
an area covered by water daily. 
The zone of beach plants is the area between high daily tide and 
the limit of storm tide. On Penikese this is indicated by piles of eel 
grass often amounting to a foot or two in thickness on sheltered 
beaches. On this eel grass and on sheltered beaches generally grows a 
group of plants which are able to endure the severe conditions. 
The following plants appear in this group: 
Ambrosia artemisiifolia Lathyrus maritimus 
Chenopodium album Ammodenia peploides 
Cakile edentula Sonchus oleraceus 
Rumex crispus Datura Stramonium 
Ammophila breviligulata (on mud 
flats) 
These plants are found only on the beaches formed of fine sand in 
sheltered positions (plate 146, upper and middle views). A second 
type of beach is found on the exposed portions of the island on the 
north and west shores. Here the fine part of the glacial till of which 
the island is made has been washed away, leaving rounded stones of 
varying sizes from huge boulders to small pebbles. This type of beach 
supports practically no vegetation. 
The tension line between the beach plants and the inland forms 
extends all around the island. It is best shown on the neck where the 
zone of the south shore merges with that of the north shore to form 
a band the width of the neck. The dominant plant in this zone is 
Achillea Millefolium, which with its associates shows a considerable 
dwarfing on this exposed pebbly neck. Along with the Achillea occur 
also: 
?By Margaret F. Shaw. 
