V. GELIDIACE.E. 113 



sub-simple branches are densely clothed with irregularly inserted ramuli. In 

 Suhria and Ptilophora the branches are flattened, traversed by a distinct midrib ; 

 in the other genera, they are either cylindrical or compressed ; in the latter case 

 thickened in the middle, but witliout a perfect midrib. The structure of the frond 

 varies considerably in minor points, chiefly in the greater or less development of 

 cylindrical, axial cells. In the typical genus Gelidium the whole structure is very 

 dense ; the axis composed of a bundle of very slender cylindrical cells, associated in 

 filamentous series, and very strongly soldered together ; the periphery, of minute 

 roundish cells formed into vertical filaments ; and the intermediate stratum, of 

 seriated cellules having a direction partly longitudinal, but curving outwards 

 towards the periphery. The whole frond, therefore, is constructed of confervoid 

 filaments strongly glued together. In Eucheuma the axial and peripheric strata, 

 both well developed, have an arrangement not dissimilar to that of the same regions 

 in Gdidmm, but the intermediate layer is composed of polygonal cellules forming a 

 honey-combed substance. In SoUeria the axial stratum is very lax, in the young 

 frond made up of a few distantly set anastomosing and interlaced filaments, but 

 gradually with age it becomes dense ; still it never attains to the closely interwoven 

 and firm axis of Eucheuvia ; the peripheric layer is very thin, and the intermediate 

 stratum of honey-combed cells forms the larger portion of the frond. At length in 

 Hypnea we find an axis reduced to a few slender filaments or even to a single fila- 

 ment ; a periphery of one or two rows of minute cellules, and a frond almost 

 entirely constituted of the honey-combed cells belonging to the intermediate 

 stratum. All these varieties of structure appear to constitute a regular series of 

 successive degradations ; and though in comparing together Gelidium with Hypnea 

 the difference is very great, yet when these genera are looked upon as the extreme 

 types of a natural group in one of which the axial, and in the other the inter 

 mediate stratum is in excess, I think it will be seen that they are brought together 

 through Euclieuma and SoUeria, which exhibit axial and intermediate strata in 

 various proportions. In external habit, too, and in substance, there is considerable 

 resemblance between the Gelidia and the HypnecB. 



If, in like manner, we compare the structure of the pericarp and its contents in 

 these genera, I think we shall find a greater siinilarity than is apparent on a cursory 

 \'iew ; taking, as before, Gelidium to express the most developed, and Hypnea the 

 least developed type of the Order. In all the genera the spore-threads have few 

 articulations, but one or at most two or three spores ripening on each thread. 

 The spores are sometimes pyriform, sometimes roundisli, slightly drawn out at one 

 end, but there is no very mai-kcd dissimilarity. The tendency is towards the pro- 

 duction of pyriform spores, and is more strongly evidenced in some than in others. 

 The great differences are found in tlie placentation, and these are so marked as to 

 have led Professor Agardh to place in three Orders the genera which I propose to 

 unite under one. To assimilate the placentation we must bear in mind that the 

 placentte originate in this group not at the apex of the mass of axial cellules, or 

 of a lateral branch from the axis, as is the case in the Sphcerocoecoidece, Lawenciacece, 

 and Rhodomelacece, but from the sides of the axial filaments themselves, which are 

 continued through the cavity of tlie conceptacle from its base to its apex. In most 



VOL. IV. — ART 5. Q 



