9 
The contrast between Zones 3 and 4 lies in the increasing 
stature and number of individuals of the species present in 
Zone 4 compared with Zone 3, rather than in any great 
change in the composition of the flora. In Zone 4 the rock 
surface in between the pools is well covered by a carpet of 
vegetation. <A striking feature is the dominance of 
Himanthalia lorea. with which is an admixture of Fucus 
serratus. The undergrowth is chiefly made up of 
Cladophora rupestris, Cladostephus spongiosus, Chondrus 
crispus, Laurencia pinnatifida, Chylocladia spp. and 
Plumana. The Himanthalia zone passes insensibly into 
the Laminaria digitata zone with a similar undergrowth. 
This level marks the limit of the littoral zone, but at 
extremely low tides, especially during quiet weather, the 
student may have an opportunity of penetrating into the 
Laminaria Cloustont zone, where a search among the 
epiphytes clustering on the stems of the large Phaeophyceae 
will yield samples of many of the more delicate Rhodo- 
phyceae which have grown under optimum conditions. 
The flora of this area will repay systematic visits at 
different times of the year as it shews well marked seasonal 
successions. (See section on Periodicity.) Examination 
of the flora in the spring or early summer will illustrate the 
lengths to which epiphytism may be carried as a conse- 
quence of successive invasion of plants into the area. One 
striking feature is the persistence with which Lithophyllum 
and Corallina are found as the basal layer in the successive 
“storeys ’’ of vegetation. They may be completely over- 
laid and eventually quite hidden by superposed vegetation 
but a little investigation will reveal them, even though the 
bases of larger algae may spread over the initial coralline 
carpet. It would appear that non-coralline algae do not 
readily attach themselves to the bare limestone surface 
but it is not yet apparent by what means the Lithophyllum 
and Corallina overcome this difficulty. 
