excretory tubules that are ciliated at one end and closed at the other.(empty) reduction division: The production of gametes with half the chromosomal complement of the original cell; sexual reproduction. rhizome: An underground stem that tends to grow parallel to the surface. secondary growth: Increase in the diameter of stems and roots of many plants, particularly "dicots". sorus (pl. = sori): A collection sporangia or spores, as is often found on the underside of fern fronds. sporophyll: Spore-producing leaves that bear sporangia and are specialized for reproduction. sporophyte: The diploid (2n) stage of an organism that possesses alternation of generations in which hapoid (1n) spores are produced. stolon: A stem that grows along the surface of the ground.