Noun and の adjective
ほうし
キノコの胞子はとても小さいので、肉眼で形は分かりません。
Because mushroom spores are so small, their appearance can’t be understood with the naked eye.
The child of a cell is a spore. They're just as tiny and they're floating all around you right now.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.