The curiously shaped _Acetabularia mediterranea_ grows gregariously on limestone rocks, and shells of mussels in the Mediterranean; it resembles a minute umbrella with a small stem, sometimes as much as nine centimetres in height, and a shade which may be more than one centimetre in diameter. The cell-membrane is thick, and incrusted with carbonate and oxalate of lime. Only the lower, root-like part of the thallus, which penetrates the calcareous substratum survives the winter, and may grow up into a new plant. The sterile leaves, which drop off early, are dichotomously branched and formed of cylindrical cells separated from each other by cross-walls, but they are not grown together. The shade is formed by a circle of 70–100 club-shaped rays (fertile leaves) grown together, in each ray 40–80 aplanospores are formed, which become liberated at the breaking of the shade, and later on are changed to gametangia (compare _Botrydium_) which open by a lid and allow a large number of egg-shaped gametes with two cilia to escape. Gametes from various gametangia conjugate with one another; the product of the conjugation swarms about for some time, rounds off, and then surrounds itself with a cell-wall. The zygote germinates after a period of rest and then produces a sexual plant. The aplanospores (gametangia) thus represent the sexual generation. Class 7. =Characeæ.= The thallus has a stem with nodes and internodes; and whorls of leaves, on which may be developed the antheridia and oogonia, are borne at the nodes. Vegetative reproduction by bulbils and accessory shoots. Zoospores are wanting. The antheridia are spherical, and contain a number of filaments in which the spirally coiled spermatozoids, each with two cilia, are formed. The oogonium is situated terminally, and is at first naked, but becomes later on surrounded by an investment, and forms after fertilisation the so-called “fruit.” The oospore, after a period of rest, germinates by producing a “proembryo,” from which the young sexual plant arises as a lateral branch. The Characeæ are distinguished by the structure of their vegetative system as well as by the spirally-coiled spermatozoids, and stand as an isolated group among the Thallophytes, of which, however, the Siphoneæ appear to be their nearest relations. They were formerly, but wrongly, placed near the Mosses. The class contains only one order, the Characeæ.