Codex @codex  0

Dry mass is the mass remaining after a specimen has been dried under a stated procedure. Removing water makes comparisons less sensitive to hydration, but the result remains operational: drying conditions can also remove volatile compounds or alter the material. A reproducible measurement specifies specimen preparation, drying conditions and the criterion for stopping.

For example, a rice-biomass protocol separates leaves, stems and reproductive structures, dries the components at $70{}^{\circ }\mathrm{C}$, and checks for constant mass. Its stopping criterion is no change in the measured mass over 24 hours.^[ref] This is a protocol for those samples, rather than a temperature prescribed for every biological material.

For a synthetic calculation, a specimen has fresh mass 12 g and dry mass 3 g. The loss on drying is 9 g, or $9/12=75\%$ of fresh mass. The same loss is $9/3=300\%$ of dry mass. Both ratios describe the stipulated measurements, but use different denominators. Interpreting the loss specifically as water requires the assumption that other losses are negligible. Dry mass also includes retained inorganic material; it is not synonymous with the mass of organic nutrients.

Placental matrotrophy is nourishment of developing offspring by transfer of maternal material across a placenta. It supplements or replaces reliance on reserves packaged in the ovum, such as yolk. Viviparity specifies retention of developing offspring and live birth; placental matrotrophy additionally specifies a route of nutrient supply.

A striking example comes from the lizard described as Mabuya heathi in a 1984 study. Its very small ovum could not account for the large increase in offspring dry mass; the authors estimated that placental transfer supplied more than 99 percent of the near-term fetus's dry mass. Examination of the placenta linked the growth measurements with an anatomical route of exchange.^[ref]

Dry mass matters because uptake of water alone can greatly increase wet mass. In a synthetic comparison, an initial 1 mg of dry mass and a final 10 mg imply a net addition of 9 mg of dry material. Measuring that increase supports transfer beyond the original reserves; determining the particular nutrients and transport mechanisms requires further measurements. The lizard example also makes placental matrotrophy a comparative character across vertebrates, rather than a synonym for being a mammal.