Exine

Exine, the outer coat or exospore of the pollengrain (microspore) of flowering plants, formerly known as the extine. Smooth in the pollen of self-pollinating and wind-pollinated flowers, in insect or bird-pollinated ones it is often furnished with spinous (echinulate) or ridge-like projections. In Pinus it is produced into two reticulately-marked air-bladders. It is either simply burst or slashed to permit the protrusion of the pollen-tube in germination, or little lid-like portions of it may be pushed off.