Sexual Reproduction of Interior Spruce (Pinaceae). I. Pollen Germination to Archegonial Maturation

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Abstract
Stages of normal sexual reproduction between pollen germination and egg maturation are described for interior spruce. These prezygotic stages were studied by light and electron microscopy in more detail than was possible in previous studies, and new observations have been made. Sperm (male gametes) formation and the organelles that accompany the sperm within the pollen tube are described. The pollen tube grows toward a lipid secretion that originates in the neck region of the archegonium. Megagametophyte development was followed from the early cellular stage through the series of mitoses that result in neck cells, the ventral canal cell, the central cell, and ultimately the egg. Maturation of the egg includes formation of modified plastids (formerly described as large inclusions) and small inclusions that are membrane‐bound regions of cytoplasm. At egg maturation, modified plastids are excluded from the mitochondria‐rich perinuclear zone that surrounds the nucleus. Small inclusions occupy the periphery of the cell so that the egg nucleus and perinuclear zone are centrally located. A second article in this series describes pollen tube entry into the archegonium, delivery of sperm to the egg, gamete fusion, and proembryo and embryo formation.