The sea weeds are usually classified according to the colour of their spores; but, since this colour generally corresponds with that of the plant itself, we may almost say that they are grouped according to their general tints. There are three main divisions:-- The _Chlorospermeæ_, or Green-spored; The _Rhodospermeæ_, or Red-spored; and The _Melanospermeæ_, or Brown-spored. The _Chlorospermeæ_ contain no colouring matter other than the chlorophyll. They are mostly small weeds, of a delicate green colour; and, although they are not particularly conspicuous on our shores, they contribute very considerably to the beauty of the rock pools, where their delicate green fronds contrast richly with the olive _Melanosperms_ and the pink and white corallines. The thallus or plant-body is very varied in form, sometimes consisting of a broad membrane, but more commonly of tufts of slender green filaments or of narrow, flattened fronds. These weeds are most beautiful objects for the microscope, and they are generally so thin and transparent that no section-cutting is necessary, nothing being required except to mount very small portions in a drop of water. In this simple manner we may study the beautiful arrangement and the various forms of the cells of which they are composed. The more delicate species will be found to consist of a single layer of cells only, while in the larger forms--the _Ulvaceæ_, for example--the thallus may be formed of two or three distinct layers, and some of the cells may be elongated into tubes. A remarkable feature of the green-spored weeds is the large size of the chlorophyll granules as compared with those of the other groups, and also the great variety of forms which these granules assume. They may be easily seen under a low power, and the examination of the weeds will show that the thalli are not uniformly green, but that the colour of the plants is due entirely to the chlorophyll granules, the remainder of the plant substance being quite colourless. If a green sea weed be placed in alcohol for a short time, it will be found that the liquid assumes a green colour, while the plant itself becomes colourless. The explanation is, of course, that chlorophyll is soluble in alcohol. The presence of starch also in the weed may be proved in a very simple manner, as follows:--Mount a small piece in water, and then put a drop of iodine solution by the edge of the cover-glass. The solution will gradually diffuse itself around the object, turning the starch-grains to a deep blue colour, and so rendering them very conspicuous under a moderately high power.