THE ALG^E 



235 



Life history. The life history of (Edogonium is therefore similar 

 to that of Chlamydomonas and Vaucheria in having an asexual 

 phase, represented by motile zoospores, and a sexual phase with 

 gametes. The asexual phase, as in the other algee mentioned, 

 enables the alga to increase rapidly during periods adapted to 

 rapid growth, while the sexual phase enables the plant, by means 

 of the highly protected zygote, to pass 

 inclement seasons without danger. 



FUCUS VESICULOSUS (BLADDER 

 WRACK) 



Zoospore.. 

 formation 



FIG. 124. Asexual reproduc- 

 tion in (Edogonium 



a, two zoospores forming, from 

 the protoplasts of two cells of 

 a filament ; b, free-swimming 

 zoospore ; c, zoospore come to 

 rest and beginning to elongate 

 into a new plant; d, young 

 plant formed from a zoospore. 

 After West 



Habit. Fucus grows along the sea- 

 shore and constitutes a conspicuous 

 part of the marine flora of these coastal 

 regions. The color is due to a brown 

 pigment secreted by the chloroplasts, 

 which masks, wholly or in part, the 

 green chlorophyll pigment which is 

 also secreted by the chloroplastids of 

 Fucus. The plant body is admirably 

 adapted to photosynthesis, since it is 



flattened like a leaf and forms, at regular intervals, air cavities, 

 or floats (Fig. 126), which buoy the plant up in the sea water and 

 expose its flattened branches to the sunlight. The plants elongate 

 by means of growing points at the branch tips, which fork in a 

 dichotomous manner, thus producing numerous flattened branches 

 all lying in one plane. They are attached to rocks or pier posts 

 along the ocean shores by means of rootlike holdfasts, which 

 serve to anchor the plant body but do not serve for the absorp- 

 tion of raw food materials, since these foods can be absorbed by 

 the entire epidermal surface. 



Fucus is much more complex in structure than Spirogyra, being 

 composed of cells differentiated into an epidermis, a brown cortex, 

 and a central cylinder, or medulla, composed of elongated con- 

 ducting cells in the form of strands or chains. Fucus is thus struc- 

 turally adapted to withstand the buffeting of the ocean waves. 



