Development of the pollen wall in Tsuga canadensis (Pinaceae)

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Abstract
In discussions of exine structural types, Tsuga is often mentioned as an exception, since no infratectal layer is present in the ektexine. The present investigation documents the formation of this pollen wall type at the ultrastructural level in T. canadensis. All layers of the exine are formed during the tetrad period, when the microspores are surrounded by a callose wall. The outer layer (ektexine) is elaborated on a fibrillar microspore surface coat, while the inner layer (endexine) is elaborated on lamellated structures. The deposition of the pretectum is followed by the appearance of endexine lamellae. In the initial stages, the two layers—pretectum and endexine—appear to be separated from each other only by a dense microspore surface coat. As additional wall materials are deposited, the tectal elements become convoluted and come to rest, in places, on the now recognizable footlayer. Upon release from the tetrad, intine formation begins and continuous accumulation of sporopollenin leads to an increase in ektexine thickness. The mature pollen wall of Tsuga canadensis, with a convoluted tectum resting directly on the footlayer, is characteristic of the genus.