Question

why do tropical plants growing have jointed stalks and coarse-grained skin​

Answer 1

Answer:

In Hibiscus , the stalk of the flower is jointed.  Lower part of this stalk is narrower than the upper part.  When flower buds fall  or after plucking of flowers, upper part of the stalk is detached from this jointed area. Generally  absciss  zone is developed at the base of petiole, pedicel, floral members, fruits etc.