THE FACTORS OF DISTRIBUTION 93 



estuaries, and seas, consists, so far as plants are concerned,, 

 mainly of microscopic representatives of this group and 

 of the" Diatoms. It is noteworthy that these world-wide 

 groups of simple organisms occur in fresh, brackish, and 

 salt waters, that they are entirely submerged, and that 

 their chief reproductive structures are free-swimming 

 ciliated zoospores and spermatozoids. These characters- 

 point to the Algae being primitively aquatic; and the 

 production of ciliated spermatozoids by the gametophyte 

 of the now mainly terrestrial Bryophyta and Pterido- 

 phyta suggest a similarly aquatic ancestry for those 

 phyla of the vegetable kingdom. 



When we turn to marine aquatics we find them almost 

 entirely Algae. There are but twenty-seven known 

 species of marine Phanerogams, of which the best known 

 are our Eel-grasses or Grass-wracks (Zoster a). No- 

 flowering plants are so completely adapted to an aquatic 

 life as are these ancient types of Monocotyledons. They 

 produce a dense network of rhizomes in the mud of 

 gently sloping shores, and bear long, narrow, ribbon-like 

 leaves and a flattened spadix, the anthers on which 

 burst under water, discharging long tubular pollen, 

 which is carried by the water to the thread-like and 

 equally submerged stigmas. 



The Algae, exclusive of the Diatoms, which, as we 

 have seen, contribute to the Plankton of both salt and 

 fresh waters, fall into three main groups. Of these the 

 green Chlorophycea are mainly fresh- water, while the 

 red Floridece and the larger, leathery, olive-brown 

 Melanophycea are almost exclusively marine. 



The marine flora shows no such geographical diversity 

 as does that of the land. Drude has, however, divided 

 it into three main " Domains," corresponding to the 

 primary divisions of the land, Northern, Tropical, and 

 Southern. The Northern, extending down to about 

 41 N. lat., is the region of Laminaria, Alarm, and Fucus. 

 LaminaricB in Arctic seas reach a length of 65-80 feet, 

 Alaria furnishes some of our edible species, while Fucus, 

 the Bladder- wracks, are familiar on all our coasts. The 

 Tropical Domain, with which is included the Mediter- 

 ranean, is the region of Sargassum and the Floridece, 

 and the Southern Domain that of the gigantic Kelps, 

 Macrocystis and Durvillcea. 



BATHYMETRICAL DISTRIBUTION. Depth is to the 



