All plants are particularly interesting during the period of fruiting, and this is remarkably the case with many of our red-spored sea weeds, which are brighter and prettier while laden with their spore-producing cells; and the collector of marine algæ should always endeavour to obtain as many species as possible in fruit, not only on account of the brighter appearance that may characterise them at this time, but mainly because the opportunity of studying the mode of reproduction should not be missed. In the rhodosperms the reproduction may be either asexual or sexual. In the former case fertile spores are produced without the necessity for any outside fertilising element, and four are usually produced in each one of the sporangia, hence they are generally known as _tetraspores_. Where the reproduction is of the sexual type, the male cells are produced singly in the terminal cells of the fronds, and since they are usually crowded together in considerable numbers, and contain none of the red colouring matter that exists in the other parts of the plant, their presence is easily observed. The female cells (_carpogonia_) are also produced on the tips of the branches, and when the male elements escape from their cells, they are conveyed passively by the movements of the water, for they have no vibratile cilia by which they are propelled, and on coming into contact with the female cell they adhere closely. An opening is then formed in the latter, and the male element enters the carpogonium, which germinates, deriving its nourishment from the parent plant, and the spores are thus formed. Lastly, it is interesting to note that the asexual spores, the male cells, and the female cells are generally produced on different plants of the same species. We will now proceed to examine some of the best known and most interesting of the rhodosperms, beginning with the order _Ceramiaceæ_, which contains a number of red or reddish-brown weeds with jointed, thread-like fronds that enclose a single tube, and which are generally surrounded by a cuticle of polygonal cells. The spores are contained in transparent berry-like sacs which are naked; and the four-parted spores (_tetraspores_) are formed in the cells of the cuticle or at the tips of the fronds. Over twenty British species belong to the genus _Callithamnion_, and nearly all of them are pretty red or rose-coloured, feathery plants that are conspicuous for their beauty. Nearly all are of small size, the largest measuring only seven or eight inches, while some are so small that they would scarcely be noticed except by those who search diligently for them. The principal features of the genus are, in addition to those mentioned above as common to the order, that the spores are angular, and clustered within a transparent sac, and the tetraspores are naked and distributed on the branches.