BIOLOGY AND NATURAL HISTORY 245 



ture falls below 42°. Since temperature of gelation is related to rate of 

 cooling, agar cooled rapidly forms a gel at a lower temperature than that 

 cooled slowly. Gracilaria agar cooled rapidly may not gel above 50° whereas 

 the same sample will often gel if placed in a constant temperature oven at 

 63°. Samples of Gracilaria agar have been observed that did not begin to 

 gel, even with slow cooling, above 45°. These came from seaweed collected 

 before September. 



Slight differences have been noted between agar from G. confervoides and 

 G. joliijera of North Carolina, but a detailed comparative study has not 

 been made. Foliijera agar has a decidedly higher viscosity when liquid than 

 does confervoides agar. 



Agar from Gracilaria collected late in the season has a higher gel strength, 

 higher temperature of gelation, absorbs less water when soaked, and is 

 extracted from the seaweed with greater difficulty than that from material 

 collected early in the season (June, July, August). As the season progresses, 

 there is a slight decline in agar yield. 



The extractive from Hypnea musciformis differs from that of other sea- 

 weeds in that there must be a solute present that serves as a gelling agent. 

 Pure Hypnea extractive in distilled water apparently will not gel, but if a 

 salt or other solute is present, a gel is formed with properties related to the 

 nature and amount of solute or mixture of solutes. Thus important prop- 

 erties of Hypnea agar, namely, gel strength, temperatures of gelation and 

 melting, and viscosity of melted solutions can be controlled. Salts have been 

 used for many years to increase gel strength of Irish moss extractive, and 

 are also essential, apparently, to the formation of a gel. 



Since the properties of Hypnea agar can be controlled independently of 

 each other by varying both concentration of agar and concentration and 

 nature of solute, it can be adapted to almost any conceivable use in which 

 the presence of a solute is not objectionable. (Agar is rarely used in distilled 

 water.) Under certain circumstances, Hypnea agar meets U. S. P. specifica- 

 tions. With Gelidium, Gracilaria, and other agars, physical properties are 

 fixed within a narrow range. 



Hypnea agar in a concentration of one per cent, for example, can be 

 made to have a gel strength double that of any other known agar in the 

 same concentration. By virtue of its controllable physical properties, a long 

 list of new uses for Hypnea agar should be forthcoming, particularly uses 

 for which no other agar is suited. A more detailed discussion of Hypnea 

 extractive is given in Bulletin 3 of the Duke Marine Station, and by Humm 

 and Williams (1948). 



