| PSI | Paleozoic plants |
|---|---|
| PSI | Paleozoic simple dichotomously branched plants of Europe and eastern Canada including the oldest known vascular land plants |
| PSI | any plant of the order Psilophytales: a savannah plant |
| PSI | any plant or fossil of the genus Psilophyton |
| PSI | whisk ferns |
| PSI | falling out of hair |
| PSI | small family of lower ferns having nearly naked stems and minute scalelike leaves |
| PSI | lower vascular plants having dichotomously branched sporophyte divided into aerial shoot and rhizome and lacking true roots |
| PSI | whisk ferns |
| PSI | type genus of the Psilotaceae |
| PSI | pantropical epiphytic or terrestrial whisk fern with usually dull yellow branches and minute leaves |
| PSI | a large bee that resembles the bumblebee but lacks pollen-collecting apparatus and a worker caste |