Zygospore (zǐg'ō-spōr) (in plants), a spore formed by the union of two similar gametes, as in some of the lower Thallophytes (Spirogyra, for example). It merely is a sexual spore or oöspore, but the name is convenient in indicating the nature of the gametes. The word "zygote" is sometimes used instead of zygospore.