The leaf mesophyll cells of an angiosperm contain 24 chromosomes....

Leaf mesophyll cells are somatic cells, which means they are diploid ($2n$).

Given $2n = 24$, the haploid chromosome number is $n = 12$.

Synergids are cells of the female gametophyte (embryo sac) and are haploid ($n$). Thus, the number of chromosomes in a synergid is $12$.

The zygote is formed by the fusion of a haploid male gamete and a haploid egg cell, making it diploid ($2n$). Thus, the number of chromosomes in the zygote is $24$.

The primary endosperm nucleus (PEN) is formed by triple fusion (one haploid male gamete fusing with a diploid secondary nucleus), making it triploid ($3n$). Thus, the number of chromosomes in the PEN is $3 \times 12 = 36$.

The respective chromosome numbers are $12$, $24$, and $36$.

Answer: 12, 24, 36