pollen tube vs plant structure

pollen tube and plant structure both are nouns.

Word	Noun	Adjective	Verb	Adverb
pollen tube	Yes	No	No	No
plant structure	Yes	No	No	No

As nouns, plant structure is a hypernym of pollen tube; that is, plant structure is a word with a broader meaning than pollen tube:

pollen tube: (botany) a slender tubular outgrowth from a pollen grain when deposited on the stigma for a flower; it penetrates the style and conveys the male gametes to the ovule
plant structure: any part of a plant or fungus

Other hypernyms of pollen tube include plant part.

pollen tube (noun)	plant structure (noun)
(botany) a slender tubular outgrowth from a pollen grain when deposited on the stigma for a flower; it penetrates the style and conveys the male gametes to the ovule	any part of a plant or fungus