PROCESSING MARINE PLANT PRODUCTS 241 



only a few of the more important ones will be mentioned, such as their 

 requirements as to light intensity and temperature, their size of growth, 

 and their length of life. 



Light. Possibly owing to color and consequent varying ability to absorb 

 light, green, brown, and red algae, when they grow in the same areas, 

 tend to grow in zones. The green are found in the shallowest water, the 

 red in the deepest, and the brown in the intermediate depths. 



The requirements of algae for light limits the maximum depth at which 

 they may be found to about 100 fathoms — even at low latitudes and in 

 the clearest water. Under less favorable conditions of latitude and water 

 clarity, the maximum depth may be limited to 15 fathoms or even less. 

 Because the depth increases rapidly along many sea coasts, the area in 

 which algae are found is restricted. 



Temperature. Different varieties of algae flourish best at different tem- 

 peratures. Although red algae, for example, grow in cold water, they are 

 most abundant in tropical waters. In contrast, brown algae are found 

 primarily in colder water. The beds deteriorate and the plants die w^hen 

 surface temperatures reach 75°F. 



Size. The green and especially the blue-green varieties tend to be so 

 small as to be difficult to harvest. Thus it is the brown varieties and the 

 red ones that are of commercial importance. Brown algae may grow to 

 exceptional size, some specimens attaining a length of 165 feet or more. 



The term ''seaweed" is applied to marine algae exclusive of the minute 

 forms, and the term ''kelp" is applied to the larger of these seaweeds. 



Life. Some seaweeds are annuals, some are biennials, and some are 

 perennials. This factor of length of life is important as it determines the 

 number of times a given species of seaweed can be harvested in a year 

 and how well it will withstand continued intensive harvesting. 



Constituents. As might be expected, the principal constituent of sea- 

 weeds is the carbohydrate called cellulose. Among other carbohydrate 

 constituents are manitol, laminarin, alginic acid, and fucoidin. Manitol 

 is an alcohol containing six OH groups. One use for it is in the manufacture 

 of an explosive which is produced by nitration of the OH groups and which 

 thus is similar to nitroglycerine. Laminarin is a reserve carbohydrate that 

 is analogous to starch in land plants. Its potential uses are as a blood 

 anticoagulant and as a substitute for plasma. Alginic acid is a compound 

 of major commercial importance which will be considered later. Fucoidin 

 is a polysaccharide sulfate ester, which so far has not found wide use 

 commercially. 



In addition to carbohydrates, seaweeds contain proteins, fats, minerals, 

 and vitamins. Although they are low in fats, some varieties contain a fair 

 amount of protein, and they are all rich in minerals. Seaweeds are also 



