No zygote is formed if the the number of chromosomes within the cells did not change or remain the same i.e. diploid because for the production of zygote, half number of chromosomes are required in both male and female sex cells or gametes. If the chromosomes are half in both gametes so they fuse with each other forming zygote having diploid cell so we can conclude that for the happening of fertilization process, half number of chromosomes in gametes is necessary if not, no fertilization occurs.
Mesophyll is not a tissue technically speaking. It's just a place, a specific area. Or you can even think of it as the set of all tissues in a given area.
