V. RHODOSPERME.E. 3 



nucleus, as well as the particular nucleoli, is surrounded by a gelatinous or sub- 

 membranaceous hyaline periderm, derived from the cell walls of the transformed 

 cells. 



The classification adopted in this memoir being based on the structure of the 

 nuclei, the student will do well, if possible, to master the principles of development 

 now explained. In many cases the structure may be easily seen b}^ viewing under 

 the microscope longitudinal sections of the conceptacle, particularly those of the series 

 DesmiospermecB, where the changes between ripe and unripe fruit are less marked 

 than in the Gongylospermece. In such plants as Polyaiphonia and Laurencia no care- 

 ful cutting is necessary, simply bruising the conceptacles between two glasses being 

 sufhcient to expel the nucleus. In tlie Sphcvrococcoidea; and in Gelidium, &c. thin 

 cuttings easily made after a little practice under a simple lens, must be viewed by 

 transmitted light. In the Gongylospermece the proper structure of the nuclei is 

 often difficult to be ascertained, owing to the confused crowding of spores which 

 results from the continual self-division of the fertile cells. It may be necessary to 

 examine thin cuttings of various ages in order to determine whether the nucleus be 

 formed from one or many mother-cells. But in most cases where the nucleus is of a 

 large size, very dense, with indications of internal septa, such an origin may be taken 

 for granted. Instances of simple nuclei or favelke favourable for examination occur 

 in Callithamnion, Grateloupia, Gloioslphonia ; and of compound nuclei or favellidia 

 in Chondrus, Gigartina, Calophyllis, and Cystoclonium. 



2. Tetraspores {SphcerosiJorce, J. Ag. Tetrachocarpia, Kiitz.) are commonly formed 

 in compound fronds by the evolution of some of the cells of the cortical layer, or 

 in the simple, filiform kinds by the transformation of the ultimate ramuli. They 

 are either dispersed equally through the surface cellules of the whole frond ; or 

 confined to the ramuli ; or gi'ouped together in definite spots called sori ; or lodged 

 in external wart-like excrescences {nemathecia) ; or in proper leaflets {sj>oro2>hylla) ; 

 or in ovate or lanceolate, podlike receptacles [stichidia). AVherever placed, tliey 

 agree in structure ; each tetraspore containing, within its hyaline, gelatino-membra- 

 naceous coat or perispore, a dense, four parted, deeply coloured endochrome, dividing 

 at maturity into four (rarely eight or more) sporules. The mode of division of the 

 nucleus varies in difterent species, and should be carefully observed by the student, 

 as generic distinctions are sometimes grounded on this character. In some tetra- 

 spores the mass is quartered by two lines crossing each other at right angles in the 

 centre ; such are called cruciate tetraspores. In others the mass splits into four 

 unequal parts, three of which only are visible on the lateral view, divided from each 

 other by three lines radiating from the centre ; such are called tripartite, ternately 

 parted or triradiate tetraspores. Again, there are others, of an oblong or cylindri- 

 cal form, divided by three parallel, transverse lines into annular portions ; and 

 such are called zonate, annular, or transversely parted tetraspores. 



The natural arrangement of the Rhodospeujik.e in Orders and genera offers 

 many more difficulties to the student than that of the Melanospeume.t., owing to 

 the greater complication of the organs of fructification ; the minuteness of the parts 

 which require to be dissected that their proper structure may be understood ; the 

 much more varied forms which \.\\q frond assumes; but especially the great diversity 



B 2 



