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for algae. However, they bear flowers essentially like those of all 

 terrestrial flowering plants and produce seeds as do all phanerogams. 

 They are true flowering plants which have merely become adapted 

 to a marine existence. 



A very cursory examination of literature reveals at least ten 

 genera of these marine phanerogams and almost fifty species. 

 These genera represent five different families of as many orders of 

 primitive phanerogams : Cymodoceaceae [Najadales], Ruppiaceae 

 [Potamogetonales], Zosteraceae [Aponogetonales], Posidoniaceae 

 [Jiincaginales], and Hydrocharitaceae [Butomales]. A more inten- 

 sive search of the literature would doubtless reveal more species and 

 a thorough search of the sea-coasts of the earth would probably 

 bring to light many as yet undiscovered and undescribed forms. 



Probably the most famous of these marine flowering plants is 

 Zostera marina, known popularly as "sea-wrack", "grass-wrack", 

 and "eel-grass". This inhabits the coastal waters of Europe, Asia 

 Minor, North America, and eastern Asia. It grows wholly under 

 water, its long jointed stems rooting at the nodes and creeping along 

 in the sand or mud of the ocean floor, sending up erect slender 

 branches, which produce long and narrow, alternate, grass-like, 

 ribbon-shaped leaves. The flowers are unisexual, naked, sessile, and 

 hidden in a sheath. It is gathered in great quantities along with 

 marine algae for manure. When dried it is extensively used for 

 packing and by upholsterers under the name of "alva marina". 

 Another species, Z. nana, inhabits the North Sea, Caspian Sea, 

 Black Sea, Mediterranean, and the water around the Canary Islands. 

 Other species inhabiting European waters are Z. hornemanni and 

 Z. dubia, while off the coasts of Japan is found Z. japonica, in the 

 Tasman Sea and Antarctic Ocean about Australia is found Z. tas- 

 manica, and off the coasts of western North America Z. pacifica. 



An interesting feature of the eel-grass is that it seems periodi- 

 cally to be attacked by some micro-organism which spreads with 

 almost unbelievable rapidity and wreaks great destruction to the 

 plants not only on one coast, but often throughout its distribution. 

 Sometimes the attacks are local and then may in part be due to 

 sudden changes in the salinity of the water or some other feature 

 of the environment, but the literature is filled with references to 

 periods when the plant was singularly scarce. At least twice in 

 recorded history this scarcity was world-wide. The first of these 



