240 THE VEGETABLE WORLD. 
in the North Sea it grows so abundantly that it is used for 
domestic purposes, such as roofing houses. It is also cut twice a 
year in some places in order to extract soda from its ashes after 
burning it. The plane part of its bifurcated frond is sown with 
Fig, 315. Fucus Vesiculosus, 
globulous vesicles or air-bags, which are probably ine ee 
sustain the plant in the water and perform the functions © 
air-cells in fishes. Certain mamelous tubercles invest the © 
tremity of the bifurcations of the frond. Jant 
If the water is withdrawn from certain portions of the P re 
at the time when the tubercles are fully developed, 1 38 se . 
times observed that their orifice is closed by a drop of Be 
liquid, while other portions of the same plant present m 
circumstances a sort of secretion, no longer red, but olive. vice 
This change of colour seems at first glance to indicate a P a 
