Sexual Reproduction of Interior Spruce (Pinaceae). II. Fertilization to Early Embryo Formation

Abstract

Stages of normal sexual reproduction between pollen tube penetration of the archegonium and early embryo formation are described for interior spruce. These pre‐ and postzygotic stages were studied by light and electron microscopy in more detail than was possible in previous studies, and new observations have been made. The pollen tube tip penetrates between the neck cells toward the deteriorating ventral canal cell, and sperm (male gametes) and accompanying cytoplasm are released into the egg. A carbohydrate plug forms around the remnant cell walls of the pollen tube and ventral canal cell and appears to seal the archegonium against penetration by later arriving pollen tubes or from release of egg cytoplasm. One sperm fuses with the centrally located egg nucleus, and the other remains in the peripheral cytoplasm of the egg. Modified plastids of the egg deteriorate, and mitochondria of the egg accumulate around the zygote nucleus. Male plastids and mitochondria cluster near the zygote nucleus. Successive mitoses result in a four‐nucleate proembryo. The nuclei and surrounding neocytoplasm migrate to form a single chalazal tier of proembryo nuclei. All plastids in the proembryo appear paternal in origin, and all mitochondria appear maternal. Cell divisions and cell wall formations result in a four‐tiered proembryo. Elongation of the suspensor tier forces the early embryo into the megagametophyte.