Seasonal Development of Growth Layers in Fraxinus campestris and Acer saccharinum

Abstract

1. The aim of this study was to follow the course of the seasonal development of the xylem in Fraxinus campestris Britton and Acer saccharinum L. at Lincoln, Nebraska, by the histological method. 2. In the trunk of the ash, one foot high, xylem formation had started just prior to April 15, when the trees were in full bloom, and before any leaf enlargement had occurred. By April 27 the innermost row of tracheary tubes had reached maturity; by May 4 all of the large tubes were fully grown; and by May 14 all the spring wood had been formed and lignification was proceeding rapidly. The formation of new cells had decreased somewhat on June 13, and considerably so on June 28. By July 15 the summer growth had almost ceased, and over 90 per cent of the xylem was completely lignified. Renewed cambial activity was producing a few cells on August 15 on one side of the trunk, but on the other side it had not started again. On September 14 the formation of new xylem cells had ceased and lignification was complete to the cambium. 3. In the trunk of the maple, one foot high, xylem formation started in one tree just prior to April 20, and in another tree about April 27, when the leaves were about half grown. The production of new cells was very active throughout May and June, but on July 15 it had decreased considerably. By August 1 most of the season's xylem had been formed, and over 90 per cent was completely lignified. On August 15 lignification was complete to the cambium and no new cells were being produced, but on September 14 cambial activity had been renewed and new xylem cells had developed, after growth had apparently ceased for the season. By October 14 cell formation had ceased and the xylem was completely lignified. 4. On April 27 the xylem development in the ash was similar in the base of the trunk and in the 1924 and 1925 portions of the twigs, except that lignification was slightly more advanced, and the tracheary tubes were smaller and more numerous in the twigs. In the maple on April 27 xylem formation was much further advanced in the young twigs than in the trunk, and it decreased progressively as the base was approached. 5. Cambial activity, as measured by the width of the cambium layer composed of the narrow, thin walled cells, showed no correlation with precipitation. With mean temperature, however, a direct correlation appeared to exist in the spring, until about 60⚬ F. was reached, after which there appeared to be an inverse correlation.