96 COMMON FUCI. 



different times, in a very different depth of water, it varies 

 greatly in size. The specimens found near high-water- 

 mark are small, and generally without air-vessels, these 

 organs not being required to float the plant in shallow 

 water : while all that grow in deep water are abundantly 

 provided with them, and have fronds several feet in 

 length, that stand erect in the water, buoyed up by the 

 air-vessels. About the level of half-tide a fourth species 

 of Fucus makes its appearance, Fucus serratus, distin- 

 guished from all the rest by its toothed margin, and 

 the absence of air-vessels. This species abounds on 

 all the rocks, to the limit of low-water, growing, like 

 F. vesiculosus, in society. These four species are all the 

 true Fuci that are common to every part of the coast, 

 and that impart to the vegetation of the rocky sea- 

 beach its peculiar olive-brown character. All of them, 

 but particularly F. serratus and F. vesiculosus, are em- 

 ployed in the manufacture of kelp, an impure carbonate 

 of soda, obtained by burning the dried stems of these 

 plants. Before the alteration of the tariff, and especially 

 in war-time, when the market was badly supplied with 

 alkali, great revenues were obtained by the owners of 

 rocky shores from the trade in kelp ; but, now that soda 

 is procured by an inexpensive chemical process from 

 rock-salt, the manufacture of kelp has been much neg- 

 lected, and has dwindled down to insignificance. At pre- 

 sent the only demand for this commodity, is from the 

 manufacturers of iodine, the chief source of that valuable 

 substance being found in the Algce of this family. It 

 is much to be regretted that a trade, once so valuable to 

 a large population on the western coast of Scotland and 



