INTRODUCTION. 5 



The fronds of different genera vary much in shape 

 and character. Some consist wholly of a flat, leaf-like 

 membrane, as Ulva and Porphyra ; others are simple 

 membranous sacs, as Enteromorpha and Asperococcus ; 

 some are cylindrical and solid throughout, while others 

 are so only in the lower part, or stem, and have ex- 

 panded leafy processes above. A few are similar in ap- 

 pearance to lichens and fungi, while others, again, are 

 scarcely to be distinguished from the rocks on which 

 they grow. 



In the higher or compound forms the frond is gene- 

 rally plant-like, and furnished with stem, branches, and 

 branchlets. The stem is in most cases composed of two 

 or more series of cells, and if these cells are all of the 

 same length, the stem has the appearance of being jointed 

 or articulated. If the cells are of unequal length, no 

 joints are apparent, and the stem is said to be inarti- 

 culate. The allied genera Rhodomela and Polysiphonia 

 are examples of these two kinds ot fronds, those of the 

 former being inarticulate, those of the latter articulate. 

 This cellular arrangement of the interior of the frond is 

 a very important element in the study of Sea- weeds, and 

 is a character much relied on in the determination of 

 orders, genera, and even species. 



In order to ascertain the nature and position of the 

 cells, it is necessary to examine thin transverse and 

 longitudinal sections of the different parts of the frond 

 under a microscope. In some genera they are uniform 

 in size, and all arranged longitudinally ; but more com- 

 monly they are divided into two principal series, that in 

 the centre called the axial series, that on the surface 

 the peripheric or cortical series. The former series is 

 generally composed of elongate thread-like cells, ar- 



