The hickories are large, deciduous trees which tend to form upright, cylindrical crowns when grown in the open. All species have pronounced taproots which securely anchor the trees, if soil conditions allow deep root development.
The wood of hickories is known for strength and resilience and is excellent for tool handles. High quality hickory is used in the manufacture of skiis, gymnastic bars, and other athletic equipment and as a flooring material for gymnasiums, roller skate rinks, and ballrooms. Some wood is used in making furniture, in piano construction, for butcher's blocks, for wall paneling, and interior trim. Hickory wood is also used for dowels, ladder rungs, and pallets (Harrar, 1958). Hickory wood is consumed in the smoking of meats and cheeses, where it imparts a distinctive flavor. Harrar (1958) reported that quality hickory lumber is in short supply, with much of the standing timber being so defective that harvest was not economical.
Heimsch and Wetmore (1939) studied wood anatomy in the Juglandaceae and found the species in Carya to be characterized by extreme ring porosity, thick walled rounded vessels with porous perforations, and shortened vessel length as compared to other genera. They concluded that Carya has attained a level of structural organization not found in the other genera of the Juglandaceae. Heimsch and Wetmore (1939) disparaged the applications of their methods to phylogenetic studies at the intrageneric level, due to a lack of adequate anatomical material or adequate criteria for distinctions. However, Kribs (1927) noted that the woods of species in section Apocarya have thinner walled vessels and fibers and are of lower density and strength that those of section Carya, an observation consistent with the reputations of the two sections in commerce. Taras and Kukachka (1970) noted that members of Apocarya exhibit apotracheal banded parenchyma in the early wood zones while members of Carya do not. Those authors also note that Apocarya shows a gradation in size of pores from early to late wood (semi ring porous), while Carya section species are more distinctly ring porous. To the extent that such structural differences influence the ease of water movement in the tree or reflect differences in duration of growth between sections, they may be involved in observed differences in graft compatibility between the sections.
Hickories have alternate, exstipulate, odd pinnately compound leaves which are aromatic when crushed. Leaflets are lanceolate to obovate with serrate edges (except in the Asian Carya sinensis (section Rhamphocarya) which has entire leaflet margins).
Flowering of Carya species is complex and has been studied in detail by Manning (1938, 1940, 1948a) and has beenrecently reviewed for pecan by Wetzstein and Sparks (1986). Trees bear male and female flowers at different locations on the same tree; male flowers are produced on slender, drooping catkins which arise from one or two pair of opposite lateral buds encased, with the shoot bud, in outer scales (although occasionally from the leaf axils in some species). Each of the lateral staminate buds will produce three catkins on a single stalk. Female flowers are borne in a spike at the end of the current season's shoot. The period of maturation for male and female flowers differs on the same tree (dichogamy); some trees shed pollen before pistillate flowers mature (protandry) while others mature pistillate flowers prior to pollen shed (protogyny). Pollen is disseminated by the wind. This type of flowering encourages genetic heterozygosity within a species (Thompson & Romberg 1985). Trees of the upland hickory species tend to commence growth in the spring and to flower prior to bottomland species, but considerable overlap in pollen shed and pistillate receptivity occurs between species (Grauke et al., 1987). This allows for the large number of recognized interspecific hybrids in the genus.
Fruit matures and falls in the autumn. The outer husk dehisces along sutures (more or less, depending on species and genotype) and either releases the hard shelled nut or falls still encasing the nut. Fruit production tends to be cyclic, with heavy crops produced in alternate years, with less pronounced cycles of longer duration in pecan (Gemoets et al., 1976).
To separate hickories (Carya) from walnuts (Juglans) in the field in any season, cut to the pith of previous season's shoots: hickories have a solid pith (left in photo); walnuts have a chambered pith (right in photo). If fruit is present, the genera can be separated on the basis of husk dehiscence: Carya spp. have husks which are dehiscent into valves while Juglans spp. have indehiscent or irregularly dehiscent (in J. regia) husks . If husks are not present, nuts can be distinguished on the basis of the position of vascular bundles in the nut. In Carya, the funicular strands of the primary septum are widely separated, and nuts have a basal plexus; in Juglans, the funicular strands in the primary septum are close together and nuts lack a basal plexus.
Warren Chatwin , Research Geneticist
USDA-ARS Pecan Genetics
10200 FM 50
Somerville, TX 77879
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