[Illustration: FIG. 64.--_Chara fragilis._ Germinating oospore (_sp_); _i_, _d_, _g_, _pl_, form together the proembryo rhizoids (_w′_) are formed at _d_; _w′_ the so-called tap-root; at _g_ are the first leaves of the sexual plant, which appears as a lateral bud.] About 40 species of fossilized _Chara_, determined by their carpogonia, are known in the geological formations from the Trias up to the present day. Class 8. =Phæophyceæ (Olive-Brown Seaweeds).= The Phæophyceæ are Algæ, with chromatophores in which the chlorophyll is masked by a brown colour (phycophæin). The product of assimilation is a carbohydrate (fucosan), _never true starch_. In the highest forms (_Fucaceæ_), the thallus presents differentiation into stem, leaf, and root-like structures. The asexual reproduction takes place by means of zoospores. The sexual reproduction is effected by the coalescence of motile gametes, or by oogamous fertilisation. The swarm-cells are _monosymmetric_, each moved by two cilia which are true protoplasmic structures, and generally _attached laterally_ (Fig. 65). The Phæophyceæ are almost entirely saltwater forms; a few species of _Lithoderma_ live in fresh water. The class is divided into two families:-- 1. PHÆOSPOREÆ: 1 Sub-Family, Zoogonicæ; 2 Sub-Family, Acinetæ. 2. CYCLOSPOREÆ: Fucaceæ. Family 1. =Phæosporeæ.= The family consists of multicellular plants, whose cells are firmly united together to form a thallus; this, in the simplest cases, may be a branched filament of cells (_Ectocarpus_), or, in the highest, may resemble a stem with leaves (_Laminariaceæ_), while all transitional forms may be found between these two. The thallus grows by intercalary divisions (_e.g. Ectocarpus_), or by an apical cell (_e.g._ _Sphacelaria_); pseudo-parenchymatous tissue may sometimes be formed by cells, which were originally distinct, becoming united together. The size of the thallus varies; in some species it is quite small--almost microscopical,--while in the largest it is many metres in length. The vegetative cells in the lower forms are nearly uniform, but in those which are more highly developed (_Laminariaceæ_ and _Fucaceæ_), they are sometimes so highly differentiated that mechanical, assimilating, storing and conducting systems may be found; the last named systems are formed of long cells with perforated, transverse walls, which bear a strong resemblance to the sieve-tubes in the higher plants. [Illustration: FIG. 65.--Swarmspore of _Cutleria multifida_.]