42. What are algae? 



Algae are primitive plants ranging in size from a single cell, which 

 can only be seen with a microscope, to the giant kelps, which grow to a 

 length of 100 feet. Algae are dominant in the sea, both in number of 

 species (approximately 6,600) and in number of individual plants. Al- 

 though algal cells contain chlorophyll and other pigments, these plants 

 do not have roots, stems, or leaves. However, some larger forms do have 

 structures which resemble these organs. 



Algae do not need roots, because they live in a solution of nutrients 

 and the whole plant can absorb water and nutrients from this solution. 

 Some algae have a holdfast that resembles a root. The holdfast is simply 

 a structure that holds the plant in place; it does not absorb water or 

 nutrients from the "soil"; therefore, it cannot be called a root. Since 

 most of the plant can absorb materials needed for sustenance and 

 growth, there is no need for an elaborate system to transport water, 

 nutrients, and food; therefore, algae do not have stems. The supporting 

 structure of kelp that resembles a stem is called a stipe; it does not 

 serve a transport function and it does carry on photosynthesis. 



Some algae have blades that resemble leaves, but these are exten- 

 sions of the plant body and are not the primary site of photosynthesis 

 as in terrestrial plants. Because the entire body of the algal plant carries 

 on photosynthesis, the blades are adapted to increase the surface area 

 to make absorption and photosynthesis more efficient. 



Photosynthesis requires light, and, since the amount of light avail- 

 able in the water is limited by suspended particles, the blades with their 

 larger surface area enable the algae to receive more of the available 

 light. When the water is very turbid, light penetration is poor and plants 

 grow only in shoaler areas. Plants with large surface areas have a better 

 chance of survival. 



Large plants do not usually grow in the open ocean, but are re- 

 stricted to water less than 300 feet deep; one exception is the sargassum 

 weed which floats in the surface layers of the Sargasso Sea. Algae in the 

 open ocean are generally one-celled forms and are limited to the lighted 

 zone (surface to approximately 600 feet). These algae are extremely 

 numerous and are referred to as the "grass of the sea" because they are 

 the very beginning of the food chain in the sea. 



46 



