XVI INTRODUCTION. 



The fructification of the algae consists of spores^ 

 either contained in conceptacles or immersed in the 

 frond^ and of tetraspores (spores when mature se- 

 parating into four parts); which are imbedded or 

 arranged in spots on the frond^ and are very rarely 

 contained in proper receptacles. Both these kinds 

 of spores are too small to be seen with the naked 

 eye 3 but the receptacles or other bodies (similar to 

 the seed-vessels of flowering plants) which contain 

 them are usually large enough to be detected with- 

 out the assistance of a magnifying glass. Some 

 plants produce both descriptions of fruits; when 

 such is the case^ the spores are described as the pri- 

 maryy and the tetraspores as the secondary fruit. 

 These terms are employed to give greater clearness 

 in the descriptions, and not to convey the idea that 

 one form is of more importance than the other, as 

 each are alike capable of producing a new plant. 

 Many species of the Fucacese are furnished with air- 

 vessels, which buoy up their fronds in the water ; 

 in the Fucus vesiculosus these vesicles are particu- 

 larly abundant, arising within the substance of the 



