INTRODUCTION. XXlll 



regard, seemingly, to the comparative perfection of the 

 structure of which it forms a part. It is often as lax and 

 as slimy in some plants of the higher tribes as in those of 

 lower organization, and some of the latter have it as firm 

 and consistent as any of the former. Thus the frond of 

 Champia and Chylocladia among Laurenciacese is filled 

 with a watery gelatine ; that of Splachnidium among Fu- 

 caceae with a loose, slimy matter ; whilst Rivularia among 

 Oscillatoriaceae has a singularly firm and consistent jelly. 

 In Mesogloia it is Very loose, investing the threads of which 

 the frond is composed with a lubricous sheath. In Gigar- 

 tina, Chondrus, &c., it is so firm as to give those plants 

 the consistence of cartilage, and in these it is immediately 

 dissolved in hot water, opening to us a curious and unex- 

 pected affinity between them and Mesogloia ; for if a branch 

 of any Gigartina be plunged into hot water it will be con- 

 verted, by the dissolving of its gelatine, into one having 

 all the characters of the frond of the former genus. Thus 

 we find that there is no fundamental difference in the 

 structure of the frond of these two apparently dissimilar 

 genera, but that one has a firmer gelatine than the other. 



The cellular tissue of Alga? presents some varieties. 

 The most common form of the cell is cylindrical, often of 

 very small diameter in proportion to its length ; and, in such 

 cases, the cells always cohere by the ends into threads or 

 filaments, bundles of which, either branched or simple, 

 form the frond by lateral cohesion. The fronds of many 

 of the simple kinds, Confervea, Ceramics, &c., consist of 

 a single thread, or string of cells or joints. Those which 

 are more compound may generally be resolved into such 

 threads by macerating small portions, either in hot water, 

 or, if that prove ineffectual, in diluted muriatic acid. If a 

 branch of a Fucus (say F. tuberculatus) , be so treated, and 

 a thin longitudinal slice be then examined with the micro- 



