During a laboratory study of plant development, a single haploid moss spore germinates and undergoes synchronous mitotic cell divisions to form a multicellular protonema.

• The first mitotic cell division takes 24 hours.
• Each subsequent division takes 8 hours.

After exactly 3 days of growth, the protonema has a total volume of 6.4 × 10^-4 mm${}^3$.