Morphology and Structure of the Walnut
C. C. LOUNSBERRY, Iowa State College
This subject, the structure of the walnut, is discussed in its relation to propagation. Catkin bearing nut trees, such as the walnut, have a refined structure that makes grafting difficult. Structure, rather than form of walnuts, suggests treatment under the headings, bark, cambium, wood, roots, pith and buds, as well as the sap that permeates them.
Bark: When the bark of the walnut is cut, as in budding, it is difficult to tie down so it will not curl and yet not strangle the bud. The wax-like covering of the bark is thin. However, the bark itself will stay green two months or more if weather is cool.
Cambium: The cambium dries quickly when exposed to air, and must be kept covered. Grafted walnuts show callus growth from the cambium, and also from the pith of stems and the endodermis of the root.
Wood: The wood of the walnut is diffuse porous, brittle, straight grained, and easily split. The wood must be cut diagonally to get sufficient tension to hold the scion in grafting. The branch grows rapidly in a short season, May 15th to July 1st in central Iowa. The upper two-thirds of the one year growth is usually light weight with pith of large diameter. The base of the one-year growth is the best for scions. Some varieties of walnut as for example the Thomas, have relatively large one-year growth and more scions can be cut from its branches than from the wood of Ohio which is small and willow-like.
Measurements taken in 1940 on 118 common black walnut seedlings planted in 1939 showed 9/16" average diameter of seedling at crown, 5/16" average diameter of pith at crown; 3/8" average diameter of seedling at top; and ¼" average diameter of pith at top; 3.26 inches average length of solid pith above crown; 2.91 inches average length of solid pith in root below crown.
Pith: Pith in the black walnut is chambered (lamellate) in the older wood, but solid in the younger, growing wood. The plates are a light brown color, getting larger in diameter toward the top of the year's growth. The leaf traces from the leaf rachis to the pith show heavier from the bottom buds of the branch than at the tip, and the pith is usually solid at the bottom of the branch.
Roots: When the nut of the black walnut germinates in the soil the lobes or cotyledons do not rise above the ground like the cotyledons of the bean but remain in the nut shell under ground, and are broken off in the growth of the seedling, the root going down and the stem rising above the ground. Where the cotyledons are broken off, the so-called crown of the walnut, two rough places appear, nearly opposite on the stem. In these rough places, two groups of buds are formed, rarely three groups.
Cytological studies at Iowa State College have not shown why there are not stem initials in the tap roots of the walnut. When the root is cut off a foot underground, root initials develop but no stem initials. The sensitivity of walnut leaf buds to water may have something to do with it.