1915 Muensclier; on Algal Associations 67 



prominent example of the relation of Fucus to its substratum is seen in 

 False Bay. Here we find a bay with a shallow sandy bottom. When 

 the tide goes out there is here exposed a bed of sand about three-fourths 

 of a mile long and almost half a mile wide. This floor of sand is almost 

 destitute of vegetation except for a few diatoms and other plants grow- 

 ing on the boulders strewn over the bed of the bay and on the raetamorphie 

 rocks along its southeast shore. The absence of vegetation on the floor 

 of this bay is not due to exposure, but to the shifting nature of the sand, 

 which makes it impossible for the algae to attach themselves perma- 

 nently. The only plants which are found here are diatoms and small 

 beds of eelgrass (Zostera marina), with an occasional plant of Ulva 

 lactuca growang upon them. Farther out where small pebbles are pres- 

 ent, Laminaria bullata and scattered plants of Phyllitis fascia are found. 

 Drifted masses of Nereocystis luetheana, Cymathaere triplicata, Costaria 

 costata, Laminaria hullata and Fucus evanescens are often strewn over 

 the bottom of the bay. The glacial boulders in the bay are covered 

 with Fucus evanescens, Gigartina mamillosa and Porphyra perforata. 

 The metamorphic rocks near the shore at low water mark have, in addi- 

 tion to the above species, an abundance of Corallina officinalis and Hedo- 

 phyllum sessile. It is evident therefore that the kind of substratum may 

 have a marked effect on the flora {Plate 13). 



In the Fucus Association are found tide pools which possess a char- 

 acteristic flora. These tide pools are left when the tide goes out, and 

 thus the plants which live in them are never exposed to the drying 

 atmosphere, but remain in the water which is heated by the sun. Ac- 

 cording to Chambers (3) the temperature of the tide pools varies from 

 that of the sea water (11.6° C.) up to 17.2° C, an extreme difference 

 of 5.6° C. This depends upon height above low tide, and exjoosure to sun, 

 among other things. Numerous tide pools were observed and a list was 

 made of the species which inhabited them. Table 2 shows the species 

 which were found in each of ten typical tide pools taken from various 

 places along the shore of San Juan Island. Prionitis lyallii and Rho- 

 domela larix were found to be the two most abundant s)iecies in the tide 

 |)ools which were studied. In table 2 the vertical columns indicate the 

 species found in each pool. Number 1 represents the most abundant 

 species in a given pool, while the highest number represents the least 

 abundant species in that pool. 



