What do tall land plants require that aquatic plants do not?
a rigid or semi-rigid stem
You would need to determine the phenotypes and genotypes of its parent plants. If one parent is tall and the other short, then the plant would be heterozygous. If both parent plants are tall, you would need to determine their genotypes as well, which may require a study of the previous two generations.
What happened when Gregor Mendel crossed homozygous tall pea plants with homozygous short pea plants?
When Gregor Mindel crossed homozygous tall plants with homozygous short plants,he got hetrozygous tall plants. Tall plants will dominate any small pea plants in the same area. The traits for tall were dominant over short traits. Perhaps both alleles can code for protein, but the dominant product is expressed in the phenotype.