450 SYSTEMATIC BOTANY. 



tacle, and themselves give exit to numerous antherozoids, each provided 

 with a couple of extremely fine cilia and containing a red granule. 

 According to our present knowledge the Fucaceae are strikingly separated 

 from the other Olive Seaweeds from the Dictyotaceae by the absence of 

 tetraspores and by the character of their anthei-idia, and*from the Phreo- 

 sporese by the absence of the reproductive zoospores and by other points 

 of organization. They appear to be allied to the Confervoid forms, 

 through Phseosporeae, more closely than to Rhodospermese ; but their 

 reproductive organs are formed on a higher type. 



Distribution. Universal ; especially found on rocks between tide-marks, 

 or, if growing in deeper water, buoyed up to the surface by vesicular 

 floats ; very large in the Southern Ocean. 



Qualities and Uses. The gelatinous substance of which the thallus is 

 composed renders some of these plants available as food for man or ani- 

 mals where better productions are scarce ; but their chief value is as a 

 source of iodine, extracted from the " kelp " or ashes, which were for- 

 merly an important source of soda also. The Fuci are also largely used 

 for manure in maritime localities. Saryassum bacciferum forms the 

 celebrated masses of " Gulf-weed " in the Atlantic Ocean. Fncus vesi- 

 culosus, the common Bladder-wrack, grows everywhere on our coast 

 between tide-marks. 



PH^EOSPOBEJE. OLIVE SEAWEEDS. 

 Class Algse, Endl. All. Algales, Lindl. 



Diagnosis. Olive-coloured or brown Seaweeds with a foliaceous, 

 shrubby, or branched filamentous thallus ; reproduced by zoospores, 

 having two cilia, one directed forwards, the other backwards, formed 

 in clavate cells or multicellular filaments, collected in more or less 

 definite groups on the cortical layer of the thallus of the larger 

 kinds, in lateral tufts or terminal on the branched filamentous 

 kinds. Illustrative Genera : Chorda, Stackh. ; Laminaria, Lamx. ; 

 Dictyosiphon, Grev. ; Punctaria, Grev. ; Desmarestia, Lamx. ; My- 

 riotrichia, Harv. ; Ectocarpm, Lyngb. ; Myrionema, Grev.. ; Leatliesia, 

 Gray. 



Structure and Life-history. This group corresponds to the tribe Lami- 

 narieae of tjhe group Aplosporese of Decaisne. The genera included in this 

 Order with highly developed thallus approximate to the Fucaceae, with 

 which they are sometimes associated ; but it has been discovered by Thuret 

 that the so-called " spores " are sacs producing zoospores, which germinate 

 and produce new plants like those of Confervoids ; they are distinguished, 

 however, from the zoospores of that group by the arrangement of the 

 cilia, which are here two in number, unequal in size, and take reverse 

 directions as they leave the body of the zoospore, resembling, in fact, the 

 form exhibited in the spermatozoids of Fucus. The size and number of 

 the zoospores are not constantly the same in the same plant ; and in dif- 

 ferent cases the organs producing the zoospores are large clavate sacs or 

 chambered filaments, the number of zoospores in a cell being either de- 



