528 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



In this system the first three of the following (main) categories are 

 annual and the remainder are perennial : ephemerophyceae {e.g. 

 Enter omorpha), being found throughout the year but often forming 

 several generations ; eclipsiophyceae {e.g. Nereia), being well developed 

 during only one part of the year and passing the remainder as a 

 microscopic vegetative form ; hypnophyceae {e.g. Dudresnaya), 

 differing from these last in that they pass the unfavourable season 

 in a resting stage ; phanerophyceae {e.g. Fucus vesiculosus), having 

 the ' frond ' perennial and erect ; chamaephyceae {e.g. Lithophyllum), 

 having the ' frond ' reduced to a crust ; hemiphanerophyceae {e.g. 

 Sargassum), having only a part of the erect ' frond ' persisting for 

 several years ; and hemicryptophyceae {e.g. Acetabidaria), having only 

 the basal creeping portion of the ' frond ' persisting. 



It is also possible, and sometimes useful, to categorize marine 

 Algae according to their particular ecological needs (such as types 

 living in pounding surf) or gross morphological characteristics (such 

 as encrusting types) or the nature of their substratum (such as 

 epiliths attached to rocks, or pelophiles growing on mud). They may 

 furthermore be divided according to their temperature requirements 

 into eurytherms and stenotherms, the latter being composed of 

 micro-, meso-, and mega-thermic species characteristic of low, 

 medium, and high temperatures, respectively, while much the same 

 can be done in connection with the factors of illumination and 

 salinity. Finally, not only do Algae show marked periodicity, 

 especially in regions where seasonal changes are pronounced, but 

 the time and duration of development of a species may be differ- 

 ent in different parts of the world, while some types which in boreal 

 regions persist throughout the year develop only in winter and spring 

 farther south. 



It should be noted that the plants composing the marine benthos 

 are very largely lithophytes, attached to rocks or boulders. The 

 more massive forms are fixed to the substratum by strong adhesive 

 disks (in F'ucaceae) or crampons (in Laminariaceae), small forms 

 usually having simpler devices. The number of species flourishing 

 on sandy or muddy bottoms is often very limited, such substrata, 

 at least in agitated water, commonly representing virtual desert so 

 far as benthic Algae are concerned, though in calm and shallow bays 

 they maybe occupied by Eel-grasses {Zostera spp.), or in warm regions 

 sometimes by other rooted Angiosperms or attached Green Algae. 

 The few Algae that flourish in such situations are usually provided 

 with root-like organs of attachment which penetrate the substratum. 



