AGAR AND RELATED PHYCOCOLLOIDS 49 



A characteristic of red algae, responsible for their economic value, is the de- 

 velopment of complex colloidal carbohydrates as cell-wall constituents. Chemically, 

 they are sulfuric acid esters of simpler carbohydrates, usually galactans. A great 

 variety of this group of substances is found throughout the phylum, and it may 

 be that each species develops a polysaccharide that is chemically different from 

 any other. Most of these phycocolloids, as they are known, will readily form a 

 colloidal solution when heated, and become a gel when cooled; examples are 

 agar and carrageenin, or Irish moss gel. A few, such as the extractive from 

 Gigartina acicularis (Wulfen) Lam., simply become more viscous when the solu- 

 tion is cooled or concentration increased. 



Edible Red Algae 



Very little seaweed, as such, is eaten in the United States. In the Orient, how- 

 ever, dried seaweed is an important part of the diet. In Hawaii natives are 

 said to utilize over 70 species of algae as food, most of which are red algae. Among 

 the poorer classes of people seaweeds constitute the principal vegetable portion 

 of the diet. In the British Isles, and to some extent on the Atlantic Coast of the 

 United States, Rhodymenia palmata (L.) Grev. and other species are washed 

 and dried and eaten as a delicacy. Dulse, the common name for dried Rhodymenia, 

 is available in cellophane packages in the large cities of the northeastern United 

 States. Seaweeds are widely used as food throughout the East Indies and by 

 the natives of New Zealand and Australia. In the West Indies their use is very 

 limited despite their great variety. Dry Gracilaria cornea J. Agardh is sold as a 

 gelling agent of foods, similar to Irish moss, in the shops in Barbadoes and possibly 

 elsewhere in the West Indies. Seaweed gels are preferred to gelatin in the tropics 

 as they set readily at room temperature (even above 90° F [32.2° C]) and, of 

 course, do not melt at summer heat as gelatin does. 



The Nori Industry of Japan. Of all the seaweeds that are dried and used as 

 food (omitting seaweed extractives), the nori industry of Japan is the largest. 

 Nori, or laver as it is known in English-speaking nations, is prepared from 

 Porphyra tenera Kjell. Laver from other parts of the world may be prepared from 

 any one of a number of species of this genus. In New England small quan- 

 tities of Porphyra atropurpurea (Olivi) De Toni and P. umhilicalis (L.) J. Ag. are 

 processed. On the Pacific Coast of the United States P. perforata J. Ag. is commonly 

 used. Some of the latter has actually been exported to China in the past (Bonnot, 

 1931). Chinese-Americans processed 300,000 pounds of laver in 1929, according 

 to Bonnot's data. 



Because of its life history, Porphyra is one of the few seaweeds adapted to a 

 form of cultivation. In most regions Porphyra appears in the fall when water 

 temperatures become low enough to meet its requirements, grows luxuriantly dur- 

 ing the winter months, and then disappears completely in the spring when water 

 temperatures rise again. It reproduces by microscopic one-celled spores which are 

 shed by the thousands from each plant. If these spores come in contact with a 

 rough surface, they may lodge against it; if the surface is in the intertidal zone, 

 germination occurs and a new plant soon appears. 



Since the vast majority of Porphyra spores fail to encounter a suitable surface, 

 the abundance of adult plants can be increased if additional surface for spore at- 

 tachment is provided in water containing many spores. The Japanese method of 



