Sexual reproduction of Pinus contorta. I. Pollen development, the pollination mechanism, and early ovule development

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Abstract
Details of development and the phenology of postdormancy cone-bud development, microsporogenesis, pollen development and pollination were similar for P. contorta var. contorta and P. contorta var. latifolia growing near Victoria [Canada], but comparable stages of development for the latter occurred .apprx. 1 mo. later near Prince George. Several developmental aspects were found which affect the reproductive potential of the species. Only 25% of the ovuliferous scales, mostly in the distal part of the cone, bear fertile ovules. Secretions formed on the ovules and micropylar arms which caused pollen to adhere to these surfaces. Pollination is by means of pollination drops which began to be exuded from the ovules .apprx. 2 wk after the conelets began to emerge from their bud scales. Pollination drops were present within each conelet for 2-4 days. At that time conelets were most widely open. Pollination drops were then withdrawn as ovuliferous scales enlarged and sealed the conelets. Pollination drop exudation and withdrawal were affected by humidity and water stress within the tree. Cells lining the micropylar canal enlarged and sealed the micropyle after the conelet closed. Pollen settled into a pollen chamber in the nucellus tip where it germinated .apprx. 2 mo. after pollination. Ovules lacking germinating pollen aborted after megasporogenesis and before free nuclear division began. If many ovules aborted within a conelet, the conelet aborted before winter dormancy. Ovules began free nuclear female gametophyte development and pollen tubes extended into the nucellus before conelets stopped developing in mid-Aug.

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