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pollen tube[′päl·ən ‚tüb]
a tubular outgrowth of a pollen grain that forms in seed plants after pollination. In its early development, it consists of a protuberance of the protoplast of the pollen grain. Covered with an internal membrane (intine), the tube emerges through a slender area of the pollen grain’s external membrane. In gymnospermous plants the pollen tube is formed in the pollen sac of the ovule; here it is embedded in the tissue of the nucellus. The pollen tube serves only as a haustorium (sucker) that receives nutrient matter (for example, in Cycadophyta and Ginkgo), or it ensures conduction of the male gametes to the female prothallium—the primary endosperm (in conifers and Gneticae).
In angiosperms the pollen grains form pollen tubes on the stigma of the pistil singly (in most plants) or in groups (in Malvaceae and Cucurbitaceae). The pollen tube, growing at its apex, emerges between the papillae of the stigma and then grows into the style and along the cells that line the stylar canal. If there is no canal, the tube grows between the cells of the stylar tissue, destroying the intercellular matter and, sometimes, the cells.
The pollen tube penetrates the ovule through the micropyle (porogamy) or, less frequently, through the chalaza (chalazogamy; in many Amentaceae) or laterally through the integument (mesogamy). In the female gametophyte—the embryo sac—the pollen tube usually enters between the egg cell and the synergid, or, upon destroying one of the synergids, it opens and releases the spermatozoids, after which double fertilization is possible.
A. N. SLADKOV