The Algæ are divisible into the green-spored[19] (_Chlorospermeæ_), the rose-spored (_Florideæ_), and the olive-spored (_Melanospermeæ_). It is in the first division that the _Protococcus_ may be placed, as also those microscopic plants called _Diatoms_ and _Desmids_. The former, the _Diatomaceæ_, are a very numerous group of minute organisms, some of which are used as test objects for microscopes. They contain in their outer coat or case a relatively large portion of silex, and their remains here and there form deposits--vast beds many feet in thickness--known as "tripoli," and used for polishing. The minute particle of their protoplasm is contained within the siliceous case. They may be entirely free, or cohere in aggregations, or be attached to a supporting surface by a slender stalk, which may ramify and bear a little siliceous case or "frustule" at the end of each branch. The desmids (or _Desmidiaceæ_) are green and devoid of silex, though their protoplasm is enclosed in hard or flexible cases, often marked with beautiful and characteristic patterns. Both diatoms and desmids may cohere together, forming more complex masses; but another creature allied to _Protococcus_ is noted for its mode of cohesion. This is the microscopic plant _Volvox_, the individuals of which cohere so as to form spheroidal aggregations, which swim about by the action of filamentary prolongations of their protoplasm, such prolongations reminding us of the pseudopodia of radiolarians and other rhizopods. Amongst these simplest plants may be also mentioned the curious thread-like organisms, which, on account of their remarkable and as yet unexplained movements, are called _Oscillatoriæ_. Another curious vegetable organism which may here be mentioned is _Vaucheria_. It is a green, thread-like plant, which may be several inches long, and which at one stage of its existence (when it is what is called a "spore") swims about by pseudopodial prolongations of its protoplasm. Some few of the _Chlorospermeæ_ are large and conspicuous organisms. Such, _e.g._, is _Caulerpa_, which abounds on warm, sandy coasts, and on which turtles browse. Though, as we shall hereafter see, it is really as simple in structure as a particle of yeast, it yet presents a very complicated external figure. Some of the great group of _Algæ_ attain enormous dimensions. Thus, _Macrocystis_ (one of the _Melanospermæ_), of the Southern Ocean, may be even 700 feet in length. Another kind, _Lessonia_, forms submarine forests, with stems like the trunks of trees. The group of _Floridiæ_ includes the delicate and elegant sea-weeds, which are amongst the most admired vegetable productions of our coasts. They are of interest, on account of various peculiarities in their reproductive processes.