Round tomatoes average between five and seven locules, or seed pockets; plum tomatoes like the san marzano have two. How did that happen? "The plum shape might have arisen as a mutation," says tomato geneticist Charley Rick of UC Davis. "Only one or two genes differentiate it from the round tomato. It apparently wasn't in any of the original varieties brought into the Mediterranean, but we really cannot determine its date of origin. There is no solid evidence that it existed before the 19th century." In a 1789 treatise on exotic plants that had been introduced into Rome, two Italian abbots described a "pear-shaped tomato which is of a more delicate and less acid taste"—possibly a forerunner of the san marzano. This tomato may have resulted from a spontaneous hybridization of two other varieties, fiaschella and fiascone, but is distinct from them—and from the roma (the plum tomato best known in the United States).
The san marzano is indeterminate in nature (it bears fruit over an extended period), with fruits grouped in bunches of five or six. It is cylindrical, with two longitudinal depressions, and has a small seed cavity that can easily be scooped out, leaving all the meat. Resistant to cracking, it ripens in 60 to 70 days in San Marzano's volcanic soil (80 to 100 days in the U.S.). More details were offered in 1920 by Ferruccio Zago, in his Nozioni di Orticultura: "The peeled tomato industry is a source of pride for Campania. People use a variety known as san marzano.... The plant can bear up to 10-12 bunches of fruit.... The skin has a bright red color and is easily removable, an indispensable characteristic for preparing peeled tomatoes. The pulp is dense and only slightly sugary in flavor...." In other words...it has less sugar!