INCREASE IN CELL WALL SURFACE AREA DURING ENLARGEMENT OF CAMBIAL DERIVATIVES IN ABIES CONCOLOR

WILSON, BRAYTON F. (U. California, Berkeley). Increase in cell wall surface area during enlargement of cambial derivatives in Abies concolor. Amer. Jour. Bot. 50(1): 95-102. Illus. 1963.Dimensions of fusiform cells (tracheids and sieve cells) and ray cells were measured from samples of the 1960 xylem and phloem increment of 5 trees felled at monthly intervals from April through July, 1960. Calculations using these measurements gave the magnitude, direction and rate of increase in cell wall surface area during enlargement. Although 14 times more tracheids than sieve cells were produced, both cell types enlarged mostly in a radial direction (up to 400%) at the same rate (20-33 X 103Au2 wall surface area/day) to the same final size. Fusiform cambial cells doubled their wall area between successive pericinal divisions. Calculations showed that / of this increase was in the radial walls of the daughter cells at a rate comparable to that in enlarging tracheids and sieve cells; the other % was from cell plate formation at an estimated rate of 187327 X 103/A2/day. Enlargement of derivatives in the radial direction largely determined the amount of increase in wall area. Besides radial enlargement, tracheids also elongated (up to 13%) and phloem cells enlarged tangentially (sieve cells up to 36%; pholem ray cells up to 60%). The relationships of enlarging tracheids and xylem ray cells are discussed, and it is suggested that slippage may occur between the developing walls. CELL WALL formation is an important part of the process of cell enlargement. The aim of this investigation was to determine the amount, orientation and rate of increase in cell wall surface area, and the relationships between adjacent cells during enlargement of cambial derivatives. As a specific example of the general process of cell enlargement, the enlargement of cambial derivatives has several characteristic features. The vascular cambium, in contrast to apical meristems, produces cells in 2 directions which differentiate respectively into secondary xylem and phloem. Also, both the cambium and its derivative tissues are composed of 2 cell systems, the fusiform cells oriented parallel to the stem axis and the ray cells oriented perpendicular to the stem axis. Thus, there is an opportunity to contrast wall formation in cells of the same origin which are enlarging in different environments and are separated spatially, and also to contrast the enlargement of 2 cell types which are already differentiated within the cambium. Abies concolor is well suited for such a study because, as in many conifers, there are few cell types in the secondary xylem and phloem. Ray and fusiform cells in the cambium produce ray and fusiform derivatives: fusiform tracheids and ray cells in the xylem; fusiform sieve cells and ray cells in the phloem. The only additional cambial derivatives are parenchyma in the 'Received for publication June 19, 1962. This study represents a portion of a thesis submitted in partial satisfaction of the requirements for the Ph.D. at the University of California, Berkeley. 2 Present address: Cabot Foundation, Harvard Forest, Petersham, Massachusetts. I wish to thank Dr. W. A. Jensen for his guidance throughout this study. phloem. The cambial cells and their derivatives are large and relatively easy to measure, fusiform cells up to 4 mm long and ray cells up to 1 mm long. In addition, derivatives at a given distance from the cambium are at a comparable stage of differentiation so that averaged dimensions are meaningful. Interest in dimensions of conifer cambial derivatives has centered on tracheids, which are the major component of the wood. Bailey (1920) noted that enlargement of tracheids was due mainly to radial expansion rather than elongation. However, because of interest in paper-making and cambial development, the literature has emphasized final tracheid length and long-term changes in the length of fusiform cambial cells, rather than dimensional changes during cell enlargement (Dinwoodie, 1961). While these studies are important to the understanding of changes in the cambium, they yield little information about the process of enlargement and increase in wall surface area of cambial derivatives. As an approach to the study of the actual process of enlargement, measurements were made of cell wall surface dimensions of fusiform and ray cells at various stages during enlargement. These measurements were used to show the magnitude, direction, and rate of the increase in wall area. Comparisons were made of the data from 5 trees to show the differences in enlargement which result in the formation of annual rings in the xylem and phloem. MATERIALS AND METHODS-Five white fir trees (Abies concolor Lindl. & Gord.), selected for uniformity, were felled at approximately 1-month intervals from April through July, 1960 (Table