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They increase in size and acquire a cell-wall, which becomes differentiated into an outer cuticular layer, or extine, and an inner layer, or intine.

Pollination having been effected, and the pollen-grain having reached the stigma in angiosperms, or the summit of the nucellus in gymnosperms, Fertilization. it is detained there, and the viscid secretion from the glands of the stigma in the former case, or from the nucellus in the latter, induce the protrusion of the intine as a pollen-tube through the pores of the grain.

The intine is uniform in different kinds of pollen, thin and transparent, and possesses great power of extension.

Intine, inner coat of a pollen grain.

The mature pollen-grain is, like other spores, a single cell; except in the case of some submerged aquatic plants, it has a double wall, a thin delicate wall of unaltered cellulose, the endospore or intine, and a tough outer cuticularized exospore or extine.