A REMARKABLE TREE-FALL AND AN UNUSUAL TYPE OF GRAFT-UNION FAILURE

IN NOVEMBER, 1941, Cornell University received fromii Norwich, New York, the basal end of the trunk of a white fir tree (Abies concolor L.) that had fallen "in a mysterious manner." The tree, an ornamental specimen grown in a cemetery, was about 40 years old, 25 feet tall, and 12 inches in diamiieter at the base. It was found fallen, separated cleanly from the stump just below the ground level. The break was nearly transverse and remarkably smooth (fig. 1, 3), suggesting to local people that it hlad been sawed down or that some animal had gnawed it down. It seemed probable that the fall was the result of a break in tissues along a line of incomplete graft union and members of the staffs of the Departments of Forestry and Botany went to Norwich and dug up the stump for further evidence of the cause of the fall. When the basal section of the trunk is fitted to the stump, a transverse furrow is evident in the bark along the line of break (fig. 2) and the trunk just above the line of break is seen to be slightly greater in diameter than the stump. The undulate surfaces of separation show, on opposite sides of the pith, a projecting hump and a concavity (fig. 1, 7). Both of these facts supported the view that the fall was the result of graft-union failure. Aniatomical study of the region of the break confirmiied this view and demonstrated the presence of a possibly new type of failure of anatomical union of stock and scion. Slight color differences of the inner bark indicated the probability that a different species was used as stock but these could not be used in identification, and wood characters also proved of no value. Bailey (1935) states that varieties and forms of firs are grafted on seedlings of the particular species, if these are available; if not, on those of .4bies balsamea (L.) AMill., A. concolor Lindl. and Gord., and A. alba Mill. (= A. Picea Lindl.; A. pectinata DC.). Though foliage forms of white fir are grafted on seedlings of the same species, it seemiis unlikely that the stock in this tree was of that species because of the differences in bark color, rate of growth, and a probable incompatibility. It is unlikely that the balsam fir was used because of the lack of vigor of growth of this species. That the European white fir was used is possible since at the time this tree was planted-about 1900-much grafted fir nursery stock probably came from Holland. Detailed anatomical study disclosed the story of the method of grafting used and the progress of growth to the fall of the tree. Perfectly preserved in the heart of the tree is the region of grafting, with the cut surfaces of stock and scion, the thread which held stock and scion together, and the callus 1 Received for publication February 26, 1945. tissue which closed the wound areas (fig. 8). The surfaces of rupture and the furrow in the bark show the position of the cambium layers of both stock and scion (fig. 7, 14-16) and the structural basis for the extraordinary break in the tree. The grafting method used was that known as "lateral," in which the scion was applied to the side of the stock. In preparation of the stock for grafting the cut was made extremely deep, well bevond the pith (fig. 10) so that less than half of the stem remained at that point. The position of the pith (fig. 8) determines the depth of the cut, as does also the absence of the pith in a transverse cut higher up (fig. 9). This doubtless accidental condition may have been in part the cause of the graft-union failure. The-scion, applied obliquely to the cut, was tied in by a linen thread (about 0.3 mm. in diam.) (fig. 11) which is preserved in position, overgrown by the later-formed xylem of stock and scion (fig. 8, 12). The lower part of the cut-surface of the stock, resin-coated, is clearly seen in the dissection (fig. 8), as is also the short oblique cut by which the top of the stock seedling was later removed (fig. 8, 12). Overgrowth by callus and xylem and phloem took place rapidly on these surfaces, both inward from the sides and downward over the top (fig. 8, 9, 12, 13). Some bark remains on this overgrowth adjacent to the scion (fig. 8). Overgrowth was so extensive and continued so long-three to six years-that it is clear that there was no close contact of the upper cut-surfaces of stock and scion; the tying-in of the scion was defective above, and the two plant-tops apparently stood apart, held together tightly only at the base of the union (fig. 11, 12). Callus development rapidly covered most of the wound surface of the stock, leaving contact with only the very base of the scion. Similarly most of the cut surface of the scion became callused. From the beginning, therefore, there was contact of vascular tissues over only a small part of the possible surface, and the cambium margins failed to unite. The failure of cambium union was probably due to incompatibility of stock and scion but may have been in part because of the early establishment on both stock and scion of callus that almost completely enclosed each wound area independently. The margins of the cambium cylinders lay close together but remained unfused. The position of the ends of the cylinders is evident from the form of the annual rings (fig. 16). The in-turned ends of the cambia formed tracheids which, similarly, were bent inward at right angles or lay horizontallv in the tree (fig. 4, 5). Thus there was no interlocking of the cells of the stock and scion, and horizontally lying vascular cells separated the stump of the tree from the trunk (fig. 4, 5, 14, 15). Contact between