A geneticist observes 10 pairs of homologous chromosomes at metaphase I of meiosis in a newly discovered species of flowering plant. How many chromosomes should be found in a microsporocyte

There are 20 chromosomes in the microsporocyte.

Explanation:

Within the pollen sac young diploid cells are originated. These cells are known as pollen stem cells or microsporocyte and they are diploid, 2n. When the microsporocyte reaches a certain degree of development, it suffers meiosis or reductional division, where during the first stage, the daughter cells get half of the chromosomes of the original cell, becoming a haploid cell, n. Each new cell suffers division again and they originate another two haploid cells. For each microsporocyte there develope four haploid cells known as microspores.

In the exposed example, there are 10 pairs of homologous chromosomes during metaphase 1. This means that there are 20 chromosomes in the stem cell or the microsporocyte. And that at the end of meiosis there will be 4 new cells with 10 chromosomes each.