=Phytol.=--This alcohol, which furnishes the characteristic ester group in the chlorophyll of plants, is a compound of very unusual composition, which has never been found in any other form or in any other type of compound which is present in either plant or animal tissues. Careful studies of its addition and oxidation products prove that it has the following structural arrangement: H H H H H H H H H H | | | | | | | | | | H-C-----C-----C-----C-----C-----C-----C-----C-----C=====C-----C--CH_{2}OH | | | | | | | | | | | CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3} As this formula indicates, the compound contains one unsaturated, double-bond linkage, one primary alcohol group, and eleven methyl groups. As has been said, this alcohol occurs nowhere else in nature, and its presence and function in the chlorophyll molecule are, as yet, wholly unexplainable. Phytol itself is a colorless, oily liquid, with a high boiling point (145° in vacuo, 204° at 10 mm. pressure). THE CONSTITUTION OF THE CHLOROPHYLLS As has been mentioned, chlorophyll _a_ differs from chlorophyll _b_ by having one more oxygen and two less hydrogen atoms in the molecule, and in having one of its nitrogen atoms in the "lactam" arrangement. These differences in structure are represented by the following formulas which are commonly used to represent the two compounds, but which do not show the arrangements of the major groups of the complex molecules: COO·C_{20}H_{39} COO·C_{20}H_{39} / / MgC_{31}H_{29}N_{3}-COO·CH_{3} MgC_{32}H_{28}O_{2}N_{4}-COO·CH_{3} |\ Chlorophyll _b_ NH-CO Chlorophyll _a_ The chlorophylls are unstable compounds, readily acted upon by acids or alkalies, and by the enzyme chlorophyllase, which splits off the phytyl alcohol group. The progressive action of acids and of alkalies in breaking down the molecule, and the products of its oxidation and reduction, have served to establish the chemical composition of the compound in each case. Because of the importance of these pigments in the whole metabolic processes of the plant, it seems to be desirable to consider the nature of these reactions in some detail, as follows: