The mature pollengrain 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.
The intine is uniform in different kinds of pollen, thin and transparent, and possesses great power of extension.
- Germinating pollengrain of Epilobium (highly mag.) bearing a pollen-tube s; e, exine; intine; abc, the three spots where the exine is thicker in anticipation of the formation of the pollen-tube developed in this case at a.
Pollination having been effected, and the pollen-grain having reached the stigma in angio sperms or the summit of the nucellus in mnos erms P gY P 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.
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.