the fatty acid radicals may be attached either in one (1) position and one (2) position, or in the two (1) positions; hence, two forms of glycero-phosphoric acid are possible, thus fatty acid fatty acid / / (A) glycerol--fatty acid (B) glycerol--phosphoric acid \ \ phosphoric acid fatty acid Again, the choline may be attached to the phosphoric acid either through its alcoholic (OH) group or through its basic (N) group, thus OH OH / / -P=O C_{2}H_{4} -P=O \ / \ \ O N[trb](CH_{3})_{3} or, O-N[trb](CH_{3})_{3} / \ HO C_{2}H_{4}OH The facts that in the arrangement (B) the central carbon atom of the glycerol would be asymmetric, and that both lecithin and the glycero-phosphoric acid derived from it by hydrolysis are optically active, prove that formula (B) correctly represents the arrangement of that part of the lecithin molecule; and there is ample theoretical and experimental evidence to prove that the choline linkage is through the alcoholic (OH) group. Hence the formula for lecithin indicating the linkage as shown above is the correct one. The fatty acids in the lecithin molecule may be different in lecithins from different sources, just as they are different in fats from different sources. Both oleic acid and a solid fatty acid have been found in the hydrolysis products of lecithin from leguminous seeds. In certain lupine seeds, the fatty acids present in the lecithin appear to be palmitic and stearic. OTHER PLANT PHOSPHATIDES Phosphatides other than lecithin are common in plants. In these, various sugars replace part or all of the glycerol as the alcoholic part of the ester. Percentages of sugar varying from mere traces up to 17 per cent of the weight of material taken, have been found in the products of hydrolysis of phosphatides prepared from vetch seeds, potato tubers, plant pollens, and whole wheat meal. Furthermore, betaine CH_{2} / \ (tri-methyl glycocoll, OC N[trb](CH_{3})_{3}) \ / O and perhaps other vegetable amines (see Chapter XII) sometimes replace choline as the basic group in the phosphatides. PLANT CEREBROSIDES