=Lycopersicin= (or lycopin) is a hydrocarbon pigment having the same formula as carotin. It is, however, brilliantly red in color, and crystallizes in a different form and has a different adsorption spectrum from carotin. It is the characteristic pigment of red tomatoes, and is found also in red peppers. Yellow tomatoes have only carotin as their skin-pigment, while lycopersicin is usually present in the flesh of the ripe fruits of all varieties and in the skin of red ones. It has been shown, however, that if varieties of tomatoes which are normally red when ripe, are ripened at high temperatures, 90° F. or above, their skins will be yellow instead of red when fully ripe. Hence, the occurrence of carotin, or of lycopersicin, as the skin pigment is determined in part by the varietal character (being different in different varieties when ripened at normal temperatures) and in part by the temperature at which the fruit ripens. The two pigments are, of course, isomers; but the difference in their structural arrangement is not known. =Fucoxanthin=, C_{40}H_{54}O_{6}, is a brownish-red pigment, found in fresh brown algæ, and in some brown sea-weeds. Its formula indicates that it is an oxidized carotin. With iodine, it forms a compound having the formula C_{40}H_{54}O_{6}I_{4}. It is unlike carotin and xanthophyll in that it has basic properties, forming salts with acids, which are blue in color. PHYCOERYTHRIN AND PHYCOPHÆIN These are the principal pigments of red and brown seaweeds, respectively. Their most characteristic difference from the pigments of non-aquatic plants is that they are easily soluble in water, and insoluble in most organic solvents, such as alcohol, ether, etc. At first thought, this would appear to be impossible, since the plants grow in water and it would seem that their water-soluble pigments would be continuously dissolved out of the tissues. The reason why this does not occur lies in the fact that these pigments exist in the cells of the seaweeds in colloidal form (see Chapter XV), and, hence, cannot diffuse out through the cell-walls. The only way in which they can be extracted from the tissues is by rupturing the cells, by grinding with sharp sand, etc., after which the pigments can readily be dissolved out by water. =Phycoerythrin= is the red pigment. It is a colloidal, nitrogenous substance, allied to the proteins (see Chapter XIII) but not a true protein compound. Hydrolysis by acids indicates that it contains leucine and tyrosine, two amino-acids which are constituents of proteins, along with other bodies of unknown composition. The colloidal solution of phycoerythrin in water has a brilliant rose-red color, with an orange fluorescence. It readily sets to a gel (see Chapter XV), so that the solution is almost impossible to filter. On this account, purified solutions of this pigment are very difficult to secure, and no satisfactory analysis to indicate its composition has yet been obtained.