The base is a small flattened disc, from which springs a frond or thallus 4 to 6 inches or more in length, having a slender subcylindrical stem, expanding fan-like into wedge-shaped segments, of very variable breadth, flat or curled, and truncate, emarginate or bifid at the summit.
The fructification[2772] consists of tetraspores or cystocarps, rising but slightly from the substance of the thallus, and appearing as little wart-like protuberances.
In cold water, carrageen swells up to its original bulk, and acquires a distinct seaweed-like smell. A quantity of water equal to 20 or 30 times its weight, boiled with it for ten minutes, solidifies on cooling to a pale mawkish jelly.
Microscopic Structure—The tissue of Chondus crispus is made up of globular or elongated, thick-walled cells. The superficial layers on both sides of the lobes constitute a kind of peel, easily separable in microscopic sections. The interior or medullary part exhibits a much less densely packed tissue formed of larger cells. The larger cavities of this tissue contain a granular mucilaginous matter, assuming a slight violet tinge on addition of iodine. In water however, the cell-walls swell up so as to form a gelatinous mass, in which separate cells can at last be scarcely distinguished.[2773] In the fresh state, its cells also contain granules of chlorophyll imbued with a red matter, termed Phyco-erythrin. But by washing and exposure to the air, these colouring substances are removed or greatly altered, and are no longer visible in the commercial drug.
Chemical Composition—The constituents of carrageen are those generally found in marine algæ, especially as regards the mucilage. This latter is insoluble in an ammoniacal solution of copper (Schweizer’s test); by the action of fuming nitric acid, it yields, in common with gum, an abundance of mucic acid. The mucilage of carrageen, like many similar bodies, obstinately retains inorganic matter; after it had three times been dissolved in water, and as many times precipitated with alcohol, we found it still to yield the same quantity of ash as the raw drug itself, that is to say, more than 15 per cent. The mucilage, perfectly dried, is a tough horny substance, of a greyish colour; it quickly swells up in water, forming a jelly which is precipitable by neutral acetate of lead.
By boiling carrageen for a week with water containing 5 per cent. of sulphuric acid, Bente (1876) obtained crystals of lævulinic acid, C₅H₈O₃, and an amorphous sugar. The former is also afforded by cellulose of pine wood and by paper.
According to Blondeau,[2774] the mucilage of carrageen contains 21 per cent. of nitrogen and 2·5 of sulphur, a statement which we are able to point out as erroneous. We find in it no sulphur, and only 0·88 per cent. of nitrogen. The drug itself yielded us not more than 1·012 per cent. of nitrogen.
When thin slices of the plant are treated with alcoholic potash, and then after washing left for 24 hours in contact with a solution of iodine in potassium iodide, they acquire a deep blue; yet, starch granules are not found in this seaweed. Lastly in connexion with carrageen may be mentioned Fucusol, an oily liquid isomeric with furfurol, obtained by boiling seaweeds with dilute sulphuric acid.
Commerce—The plant is collected on the west and north-west coast of Ireland: Sligo is said to be a great depôt for it. Carrageen of superior quality is sometimes imported from Hamburg.
The largest quantities of carrageen, sometimes half a million pounds a year, are gathered near Minot Ledge lighthouse, Scituate, Plymouth county, on the coast of Massachusetts, where a systematic process of preparing it for the market is adopted.[2775]