Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.

Sentence1: Ferns are vascular plants. 
Sentence2: when a fern grows, its size of leaves may increase.
Ferns

Sentence1: Weather Clear weather usually means good visibility. 
Sentence2: High pressure systems can cause good visibility.
good

Sentence1: Cytoplasm refers to all of the cellular material inside the plasma membrane. 
Sentence2: Plasma membrane is the cell membrane surrounding cytoplasm.

membrane