628 RELATION TO ENVIRONMENT. 



disk-like, or lobed, holdfasts, while the diatoms are attached 

 by stems of gelatine. A large number of the smaller ones are 

 epiphytes, and there are many semiparasites also among the 

 algae, as well as some fungus parasites. Very few marine plants 

 are attached to the mud or sandy bottoms. These places corre- 

 spond to deserts in the paucity of their flora. The few plants 

 found here (Caulerpa, etc.) are attached by root-like holdfasts, 

 or in the shallow places meadows of sea-grass (Zostera) are found. 

 The larger algae are almost exclusively confined to the photic 

 region (extends 30^-40^ = 100-125 feet deep), while seed-plants 

 are exceptional. 



1118. The photic, or bright light, region is divided into two 

 broad zones; the upper one is periodically exposed by the move- 

 ment of the tide and lies between limits of ebb and flow. The 

 lower zone of the photic region lies below ebb-tide. 



1. The upper photic zone. This is again divided into two 

 strata, the lower one being the most favorable for development, 

 since the members are only partially exposed at low tide. In 

 the upper stratum the conditions are austere, since the complete and 

 long uncovering exposes the plants to the danger of drying out. 

 The plants are generally somewhat stunted in growth, stout, 

 with a thick epidermis, and are sparingly branched, and they 

 are exclusively algae; example, Fucus vesiculosus. 



2. The lower photic zone. To this zone belong all the marine 

 seed-plants and the great mass of algal vegetation. This zone 

 is also divided into strata, dependent, however, upon the dimin- 

 ishing intensity of light at increasing depths. It is interesting 

 to note that in general there is a definite relation between the color 

 of the algae and the light stratum which they inhabit. The 

 green algae (Ulva, Enteromorpha, etc.) are chiefly in the upper 

 stratum, the brown algae (Laminaria, etc.) chiefly in the middle 

 one, and the red algae chiefly in the lower. Yet the relation be- 

 tween the color of the algae and the light stratum which they 

 inhabit is not as definite as was once supposed. The red algae, 

 however, are especially adapted to growing in the more dimly 

 lighted stratum of the photic region, since they are sensitive 



