A third possibility is that curliness is produced by the interaction of several factors, one a tendency to curliness inherent in the Lamb tree and the others environmental such as growth rate, nutrient supply, the nature of the soil or other such conditions.
Theoretically curly grain in walnut or any other tree is related to the nature of the growth of the cambium layer. In normal growth the cells of this layer are much elongated as seen in tangential section and are relatively straight. The nature of these cambium cells is shown in figure 2.
[Illustration: Fig. 2. The cambium of a straight-grained black walnut tree as seen in tangential section. The nature and regularity of these cells determines the nature and regularity of the cells of adjacent wood and bark (× 150).]
It is well known from studies of cambial growth that irregularities in the growth of the cambium are reflected in the irregularities in the shape and position of the wood fibers and vessels, which it forms. Ordinarily, if the cambium is wounded, the first cells formed are irregular in shape and orientation but after a wound is healed over the cambium cells resume their normal position. In parts of trees in which the grain is irregular or confused such as in the inner angle of crotches the shape of the cambium cells determines the nature of the grain beneath as shown in figure 3 (Ref. 1). This has been established also in the study of the nature of spiral-grained Douglas Fir and in various experimental work where it has been possible to change the direction or extent of the cambium cells through various experimental means. (Ref. 2)
[Illustration: Fig. 3. Section through cambium and underlying wood in a crotch of an apple tree where the grain of the wood is not straight. The shape and direction of the wood fibers or grain of the wood, and bark is determined by the shape and direction of the cambium cells that form them (X 100).]
There seems to be no doubt, therefore, that curly grain in walnut is directly related to the curly condition to be found in the cambium, which produces such curly grain. The basic question to be resolved is what makes the cambium of a curly-grain tree assume the curly or wavy character. As indicated above, one hypothesis is that several factors may be operating. For example, a tree might have the inherent capacity to produce wavy grain but would only do so under special environmental conditions. These environmental conditions might be related to rapidity of growth, water and nutrient supply, or various other habitat characteristics, which affect the nature of growth. The fact that the tree in question at Ithaca was growing rapidly might have been responsible for the failure of the curly grain to develop. There is evidence that trees with figured grain grow slowly. (Ref. 3, 4) On the other hand the specimens from the tree at Beltsville, Maryland, were from a slowly growing plant and did not show curly grain.
Another hypothesis is that development of the curly grain is dependent upon the foliage of the tree. This has been demonstrated to be true in instances where the foliage of fruit plants determines the characteristics of the growth of the trunk and roots and of the fruit itself. (Ref. 5, 6) It might be, therefore, that the failure of this particular trunk to show curly grain is related to the fact that the top of the tree at Ithaca was of another variety than the original Lamb. Possibly the foliage of the original variety producing the curly character is necessary to produce the curly grain. An argument against this interpretation is that the tree at Beltsville, Maryland, is not topworked.
It would be valuable at the present time to survey all the trees of the Lamb walnut, which are growing in various parts of the country, to see under what circumstances they may be showing the curly characteristic of the original tree. Dr. M. Y. Pillow of the Forest Products Laboratory at Madison, Wisconsin, in an unpublished report, has pointed out that it is possible to determine the curly nature of the grain by shaving off the outer bark, exposing the inner bark just outside of the cambium. Inasmuch as the same cambium cells form fiber cells both on the inside to make the wood and towards the outside to make the bark, the direction and nature of the fibers in the bark are a direct indication of the direction of the fibers underneath the cambium in the wood.
The appearance of the normal straight grained wood and bark and wood and bark of a curly grained tree are shown in figures 4 and 5. Shaving off the outer bark in this manner will not harm the trees, if it is done carefully so it would be possible to make this survey without injury to the trees. Examining a number of trees of the Lamb walnut in this way and finding that some were curly, might give evidence as to the conditions under which the Lamb walnut will produce curly grain.
Dr. Pillow of the Forest Products Laboratory, kindly furnished me with his file on curly and birdseye grained wood. In this file is a very interesting group of manuscripts and letters including a report from Mr. Willard G. Bixby reporting a trip to New Hampshire to study the occurrence of birdseye maple and also his early experiments with the Lamb walnut. The Lamb walnut trees at that time were too young to give any indication of curly grain. Other letters of interest on the subject were from Mr. J. F. Wilkinson, A. S. Colby and C. A. Reed. These letters mention the desirability of propagating figured walnut but aside from indicating that trees of the Lamb had been propagated there was no indication that curliness had developed. The first definite indication that curliness would develop in a grafted tree was reported by Mr. Wilkinson (Ref. 7) at the Norris meeting of the Northern Nut Growers Association. At that time the wood photographed in figure 1 was shown.