PLANT AND ANIMAL ASSOCIATIONS 45 



Baker (1909-19 10) states that the species of Fucaceae 

 inhabiting the upper portion of the tidal zone can resist 

 desiccation both during germination and vegetative growth, 

 and that those seaweeds that best resist this danger are of slow 

 growth. On the other hand, those that grow rapidly are 

 the least tolerant of desiccation, which factor would be 

 sufficient to cause their exclusion from the higher zones. 



Observations have recently been made regarding the 

 density of these algal groves at Port Erin, Isle of Man, the 

 weed being all cut away from a square foot of surface. In 

 one case, at half tide, where Fiicus commenced, 35 stalks of 

 F. serratus were counted, in another instance 352 stalks of 

 F. vesiculosus were found, arising from 12 " roots." On the 

 upper edge of the Laminarian Zone 6 to 8 stalks of Fucm 

 occurred and mingled with an equal amount of Laminaria ; 

 2 feet lower down the place of Fucus was entirely taken by 

 Laminaria, at times 36 stalks to the square foot (Mayne, 1918). 



An interesting example of the interaction of plants and 

 animals is furnished by a case of an excessive growth of 

 Ulva investigated by Cotton (191 1) in Belfast Lough. 

 Mussels can cover ground on which the common green 

 alga Ulva latissima cannot find anchorage until preceded 

 by mussels, which then form a suitable foundation. 

 Absence of rough water is also important as otherwise 

 the alga becomes torn. The Sea-lettuce has been proved 

 to be able to thrive in either pure sea-water or polluted 

 brackish water, to which latter it has probably become 

 adapted. In polluted brackish water Uha absorbs quan- 

 tities of ammonia, and its tissues are found to contain a 

 much larger proportion of nitrogen than those grown in 

 pure sea-water. The results of this excessive nourishment 

 are seen in an increased rate of growth ; the fronds are 

 of a different texture and darker colour, while there is 

 also a tendency for spore formation to remain in abeyance, 

 the growth being rank rather than healthy. The investi- 

 gator considered that the Ulva, which was a decided 

 nuisance in the locality, would not be suppressed until the 

 mussels were exterminated. 



