RETICULATE THICKENINGS IN SOME SPECIES OF JUGLANS

Reticulate thickenings in Juglans nigra, J. major, J. microcarpa, and J. californica (including J. hindsii) appear, under the transmitted light microscope, to be a cluster of very large or "gash-like" vessel-ray pits. The use of scanning electron microscopy shows that these so-called pits are thickenings on the inside wall of the vessel. RETICULATE THICKENINGS ("gash-like" vessel pits) have been reported in some species of Juglans, but these structures have never been adequately described or photographed. In the late 1940's Dr. B. F. Kukachka of this Laboratory noticed the presence of what appeared to be very large or "gash-like" pits in some vessels of J. nigra and found that this character could be used for separating J. nigra from J. regia because this feature was lacking in the latter. Carpenter and Leney (1952) illustrated latewood vessel elements of J. nigra that had "reticulate intervessel pitting." In their "Identification of Hardwoods" Brazier and Franklin (1961) stated that J. nigra has very large "gash-like" pits in the vessels. Also, Stark (1953) reported "gash-like" intervessel pits in J. nigra, J. major, and J. microcarpa. No mention of "gash-like" pits were made in the descriptions of Juglans by Solereder (1908), Kribs (1927), Heimsch and Wetmore (1939), Record and Hess (1943), Metcalfe and Chalk (1950), Jane (1956), and Panshin and de Zeeuw (1970). However, Kribs (1927), Heimsch and Wetmore (1939), and Record and Hess (1943) described the vessel-ray pits in Alfaroa and Engelhardia as large and "gash-like." Their use of "gash-like" refers only to the typical coarse vessel-ray pits characteristic of these two genera. A recent investigation at the Forest Products Laboratory on the wood anatomy of Juglans confirmed the diagnostic value of reticulate thickenings. Juglans nigra, J. major, J. microcarpa, and J. californica (including J. hindsii) were the only species in which reticulate thickenings were found. Observations indicated that these supposedly very large vessel-ray pits occasionally were not in contact with rays. Thus, in an effort to determine which cells subtended these so-called "gash-like" I Received for publication 23 May 1975. The author gratefully acknowledges the technical assistance of Richard Kinney with the SEM microscopy and specimen preparation. The Forest Products Laboratory is maintained at Madison, Wis., in cooperation with the University of Wisconsin. pits the SEM (scanning electron microscope) was employed. METHODS AND MATERIALS-A wood sample of J. nigra (MADw 8986, collected in the Missouri Botanical Garden Arboretum, Grays Summit, Mo., by Beilmann, BWCw 8274) and known to have an abundance of reticulate thickenings was examined with the SEM. Blocks were split or smoothed with a microtome knife to expose either the radial or transverse face. Thin sections (51O 1/m) were also examined with the SEM. In all cases the specimens were coated with gold. Latewood material was macerated in a solution of equal parts of nitric and chromic acid (Jeffery's solution), and then washed thoroughly in water. A small portion was then placed on a slide and gently teased to separate the vessels from the fibrous elements and parenchyma. With no cover slip applied, the macerated material was examined under a transmitted light microscope to find a vessel member with reticulate thickenings. After such a vessel member was found, it was maneuvered so that no other cells were near and the reticulate thickenings were facing upward or on the top surface. The specimen was allowed to dry and then coated with gold and examined with the SEM. OBSERVATIONS AND DISCUSSION-Reticulate thickenings generally occur only on the radial walls of the smaller latewood vessel members, and resemble vessel pits under the transmitted light microscope. It was originally thought that reticulate thickenings were some type of very large or "gash-like" vessel-ray pits. Figure 1 shows reticulate thickenings under the transmitted light microscope. The space between the thickenings appears to be an opening, thus the concept of vessel-ray pits. The absence of rays near the reticulate thickenings led to the examination of these structures with the SEM. In Fig. 2, reticulate thickenings can be seen on a vessel member which was separated from all other cells by maceration. Fig. 3 shows the same