The COOH groups which are attached to these compounds to form the various phyllins and porphyrins, as well as the original chlorophylls, are supposed to be attached to the C_{2}H_{5} groups in the above formulas, the different modifications, or compounds, depending upon the position in which one or more of these attachments are made. SIMILARITY OF CHLOROPHYLL AND HÆMOGLOBIN It seems to be desirable, at this point, to call attention to the remarkable similarity in the chemical composition of chlorophyll, the most important pigment of plants, and hæmoglobin, the all-important respiration-regulating pigment in the blood of animals. Hæmoglobin is a complex compound, consisting of about 96 per cent of albumin (a protein, see Chapter XIII) united with about 4 per cent of _hæmatin_, a brilliant red pigment which has the formula FeClC_{32}H_{32}O_{4}N_{4}. When treated with acids, the iron (and its accompanying Cl) is removed, and hæmatoporphyrin, C_{32}H_{36}O_{4}N_{4}, is obtained. When either hæmatin, or hæmatoporphyrin is oxidized, hæmatinic acid imide identical with that obtained from ætioporphyrin is obtained. Also, when hæmatoporphyrin is reduced, hæmopyrrole identical with that from ætioporphyrin is obtained. Thus, it would appear that the unit structural groups in hæmatin and in chlorophyll are identical; although chlorophyll may exhibit more variations in isomeric arrangement of these structural units than have been found in hæmatin. Hence, it is apparent that the only essential difference in composition between chlorophyll and hæmatin is that in the former the structural units are linked together by iron, while in the latter, the same units are united through magnesium as the linking element. Further, it is known that while iron is not a constituent element in the chlorophyll molecule, it is, in some unknown way, absolutely essential to the production of chlorophyll in plants; plants furnished with an iron-free nutrient solution rapidly become etiolated and photosynthesis stops. The following skeleton formulas have been suggested to indicate the way in which these elements are linked between the structural units in their respective compounds. -C C- -C C- \ / \ / N N N N / \ / \ / \ / \ -C \ / C- -C \ / C- M g F e -C / \ C- -C / | \ C- \ / \ / \ / | \ / N N N C l N / \ / \ -C C- -C C- Chlorophyll Hæmatin