The Kinetics of Tracheid Development in Tsuga canadensis Carr . and its Relation to Tree Vigour

Cell counts from samples taken at weekly intervals, from 14 May to 22 October 1969, in a T. canadensis stand in Massachusetts, U.S.A. , showed that the width of the annual ring was correlated with the rate of cell production and that only the least vigorous trees (c. 20 tracheids year") had a shorter growing season. The time required for completion of a cell-division cycle in the cambial zone decreased during the course of the season, from 35 to 20 days for the less vigorous trees (25-45 tracheids year") and from 28 to 10 days for the more vigorous trees (45-100 tracheids year"). The time required for the completion of radial growth of the tracheids decreased from 18 to 9 days, with no evidence of any changes with tree vigour. The actual radial growth-rate of the tracheids was constant within the range 1 -5—3 /̂ m day". The time required for deposition of the secondary cell wall increased from 10 to 50 days, with little evidence of any changes with tree vigour. The actual rate of deposition of cell wall material was about 0-15 p yr day" and seemed to show little change during the course of the season. The time period required for lysis of the cytoplasm was about 4 days, with no evidence of any changes with tree vigour and little evidence of any changes during the course of the season. I N T R O D U C T I O N Earlier work has shown the relatively long period of time taken for the completion of radial growth in developing tracheids and for the deposition of their secondary cell walls in Abies pectinata D.C. (Wodzicki and Peda, 1963) and, more recently, in Pinus sylvestris L. (Wodzicki, 1971; Wodzicki and Zajaczkowski, 1970). Skene (1969) has shown that in P. radiata D. Don a cycle of cell division in the cambial zone also takes a relatively long period of time, as do the phases of radial growth and wall deposition. The time taken to complete radial growth decreases during the course of the season, whilst the time taken for deposition of the secondary cell wall increases considerably. The aim of the present work is to extend this type of observation to Tsuga canadensis Carr. and, more particularly, to try to find out how these time periods vary with tree vigour, vigour being defined as the width of the annual ring. MATERIAL AND METHODS Material was taken from a well stocked old growth stand of Tsuga canadensis Carr. on a moist site at Petersham, Massachusetts. The trees ranged from 0*3-3 m m (over bark, at breast height) and were estimated to be 50-150 years old. 1 Present address: East Mailing Research Station, Maidstone, Kent. at H avard U nirsity on A uust 5, 2011 aob.oxjournals. org D ow nladed fom 180 Skene—Tracheid Development in Tsuga canadensis Forty-five trees, representing a wide range of vigour, were divided into three groups, trees being assigned to each group at random on the basis of an initial estimate of their vigour. Thus each group contained trees scattered throughout the whole plot and representing the whole range of vigour. At each collection one sample was taken from each tree of a group, so that during the course of the season a total of eight samples, spaced out equidistantly around the circumference, were taken from each tree at breast height. Collections were started on 14 May 1969 with the trees from group 1 and were continued at weekly intervals using trees from each group in turn (i, 2, 3, 1, 2, etc.). After the last collection (22 October 1969) there was a total of 24 collections, each with 15 samples. The samples were removed with bark intact; they were about 5 cm long, 2 cm wide, and deep enough to include both the current and the previous season's annual ring. Portions suitable for embedding were fixed in acrolein for 24 h at o °C, embedded in glycol methacrylate, sectioned transversely, and stained with toluidine blue according to the methods described by Feder and O'Brien (1968). Counts were made of the number of cells in a radial file, using an average from two radial files for each sample. Counts were made of the total number of tracheids (O) across the previous season's annual ring and the number of cells in each of five different phases of development across the current season's ring. The dividing phase (D), radial growth phase (G), and secondary wall deposition phase (W) were distinguished from one another as in Skene (1969), while the lysis phase (L) was distinguished from the secondary wall phase by the presence of the S3 layer of the secondary cell wall and the mature phase (M) was distinguished from the lysis phase by the complete disappearance of the cytoplasm. Thus the combined lysis and the mature phases of this work correspond to the mature phase in Skene (1969). For each sample the counts were added as follows: Sm = M = total number of mature tracheids in a radial file Si = M-\-L = number which have completed deposition of their