Description

Dwarf mistletoes (genus Arceuthobium) are parasitic angiosperms that infect commercially valuable conifer trees across North America, resulting in millions of dollars of damage annually. The purpose of this study was to examine the developmental processes taking place during anther development in the dwarf mistletoe species, Arceuthobium americanum, and to describe the eventual three-dimensional relationship between the anther and the columella (a region of sterile tissue within the anther). Angiosperms typically have anther walls comprised of several layers. However, the composition of the anther wall layers and aspects of layer development varies from species to species. These processes have only been superficially examined in the Arceuthobium genus. In order to obtain insight into anther wall development in A. americanum, thin sections were cut, mounted and stained on slides, and viewed with a light microscope. The data indicate that A. americanum have three distinct anther wall layers including 1) the epidermis, 2) the middle layer, and 3) the tapetum. Most angiosperms have a sub-epidermal layer called the endothecium, but this work shows that A. americanum possesses an “exothecium” similar to that of gymnosperms. The eventual three-dimensional orientation of the anther with reference to the columella was described using a combination of light and dissecting microscopy. Previously, it was thought that the columella spanned the length of the anther to such an extent that the loculus (anther chamber) was bisected; however, this study reveals that the columella does not span the length of the anther in either direction, even though it does originate centrally.