37 



£Ijf Crcatfurg of 3Snian». 



[AJLG2E 



sexual differences hare been observed; the 

 antheridia, or male organs, containing 

 bodies often closely resembling the sper- 

 matozoa of animals. In some of the spe- 

 cies fructification does not take place till 

 the threads throw out little processes, by 

 means of which a complete union with one 

 another is established, the endochrome of 

 the joint of one thread passing through 

 their lateral tube and uniting with that of 

 an opposite joint, and then forming a per- 

 fect spore. 



In many of the lower Algce, as indeed in 

 some of the higher, reproduction takes 

 place for an indefinite time by repeated 

 subdivision of the original individual. At 

 times, however, the proper fruit makes its 

 appearance, and sometimes in such an 

 anomalous form as to cause much per- 

 plexity. 



Algce are related on the one hand to 

 fuguses, and on the other to lichens. Dis- 

 tinctive characters are more easily derived 

 from their respective habits than from 

 differences of structure. 



The term Alga had formerly a far wider 

 range than at present, and it is now almost 

 entirely confined to aquatic cryptogams. 

 There is no English word which will com- 

 prise the whole. The most convenient, 

 perhaps, is that of Hydrophytes, which, 

 however, does not apply to the aerial spe- 

 cies, and is objectionable because there are 

 many plants with a submerged habit which 

 are not Algce. 



Algce are divided into three great classes, 

 each of which comprises a number of very 

 distinct groups, the more prominent of 

 which will be noticed in their proper order. 

 These three classes are characterised by 

 the colour of their seeds, which correspond 

 for the greater part with the general tint 

 of the plants. 



1. Mela>-osper3ie^:, or olive-spored: 



2. RHODOSPEE3rEiE, or rose-spored. 



3. Chlobospeeme^:, or green-spored. 



The first of these comprises the olive- 

 coloured species, which from their size and 

 abundance are so conspicuous on our shores, 

 or which float in dense masses, sometimes 

 many leagues in extent, on the surface of 

 the ocean. On our own coasts they attain 

 the length occasionally of twenty feet or 

 more, and in the genus Laminaria indi- 

 viduals are sometimes large enough to be 

 a load for a man; but this is nothing to 

 the size attained in the southern seas, or 

 even in some parts of the northern hemi- 

 sphere. Individuals of the genus Macro- 

 cystis attain a length of a hundred feet 

 or more, and Lessonia forms submarine 

 forests, the stems resembling the trunks 

 of trees. Some of the lower species have 

 nothing like leaves, and are reduced to 

 mere articulated threads, or a shapeless 

 mass. 



The second class comprises those charm- 

 ing seaweeds, remarkable for their ele- 

 gance of form, delicacy of texture, and 

 brilliancy of colour, which attract the at- 



tention of all wanderers along the coast. 

 These are often very abundant, but they 

 seldom attain any considerable size, and 

 some of them are as delicate as moulds. 



The third class contains most of the 

 smaller species, in which the frond seldom 

 assumes the form of a membrane, but is 

 more frequently reduced to a mere thread, 

 or even to single articulations. A few only 

 are conspicuous objects, amongst which 

 the genus Caulerpa is most remarkable, af- 

 fording on warm sandy coasts an ahundant 

 supply of nutritive food for turtles. Of 

 i the smaller and more obscure species, in 

 which there is often no point of attach- 

 ment, we have the most exquisite micro- 

 scopical objects, exhibiting an almost 

 inexhaustible variety of form and sculp- 

 i. ture. 



In the two latter classes, more especially, 

 | many species are so masked by calcareous 

 matter as to present the appearance of 

 corals, with which productions they have 

 accordingly been arranged. A weak solu- 

 tion of hydrochloric acid, however, soon 

 changes the fixed carbonate of lime into 

 soluble chloride of calcium, and the struc- 

 ture and fruit are then unmasked and found 

 to correspond with those of true Algce. In 

 Diatomacece, silex instead of lime is im- 

 bedded in the substance of the cells. 



Amongst the productions which appear 



upon rocks exposed to the action of the 



atmosphere, the lower Algce are often the 



I first to make their appearance. Even the 



I cold surface of snow and ice produce the 



| bright red Alga, known under the name of 



| Red Snow, while allied species appear on 



darker grounds. These gradually, by their 



decomposition, afford soil for higher 



growths. 



The larger species of Algce afford a useful 

 though coarse article of food to men and 

 domestic animals, not to mention the 

 numberless tribes which they support in 

 their own element. The Laver of our south- 

 western coasts is, however, considered by 

 many an object of luxury, though, like 

 olives, it is not in general relished at first. 

 "With use, however, it is esteemed by many 

 a most acceptable condiment. Many of the 

 rose-coloured Algce abound in gelatine, and 

 in consequence they are collected to make 

 a fine kind of glue, or as a substitute for 

 isinglass. Carrageen or Irish moss, which 

 consists, in great measure, of common spe- 

 cies of Chondrus, is a most useful article in 

 cattle feeding, when boiled and mixed with 

 other nutritious matters. Amongst the 

 Chlorosperms.besides the Laver above men- 

 tioned, a species of Nostoc is much used as 

 an ingredient in soup by the Chinese ; but it 

 seems not to have much to recommend it 

 beyond the quantity of bassorin which it 

 contains. Durvillcea util{s is employed for 

 the same purpose in Chili. The siliceous 

 coats of Diatomacece, of which the substance 

 called Tripoli is entirely composed, form a 

 capital substance for polishing, and the 

 close parallel lines of extreme fineness, 

 with which they are frequently grooved, 

 make them very useful in microscopical 

 researches as a test. 



