From the Super Mario Wiki
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The plant rests right beside a large pit full of dangerous vines. The Sun Tower lies on the other side and Mario needs a way to get there. Next to the pit is the Bubble Plant, a mysterious plant who constantly blows Bubbles into the air. Upon speaking with the plant, it tells Mario that if he gives it a Bubble Berry, it will show Mario the real reason it's called a Bubble Plant.
After Mario finds a tree that grows Bubble Berries and hands one over to the Bubble Plant, it will blow an enormous bubble that is big enough for Mario and his partners to hop in. It will fly over the large vine pit until it reaches the other end - from there, Mario can access the Sun Tower.