XVI International Botanical Congess
Needles of Pinus strobus L. were collected weekly from 20-year old trees during the period in which natural senescence and drop of two-year old needles occurs. After clearing in KOH and staining with trypan blue, needle pieces totalling 1000 mm. in length were selected randomly from each sampling and examined microscopically. The surface of the needle was screened and several phenomena including: hypersensitive responses, vesicles, hyphae of three types, and two categories of defense reaction were quantified. Hypersensitive responses varied over the study period and visible hyphae increased, possibly indicating changes in activity by resident endophytes. The nature and number of defense reactions observed during needle senescence suggests a parasitic rather than commensal relationship.