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Secondary Growth in Needle Leaves of Pinus longaeva (Bristlecone Pine) and Other Conifers: Quantitative Data

Frank W. Ewers
American Journal of Botany
Vol. 69, No. 10 (Nov. - Dec., 1982), pp. 1552-1559
Stable URL: http://www.jstor.org/stable/2442909
Page Count: 8
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Secondary Growth in Needle Leaves of Pinus longaeva (Bristlecone Pine) and Other Conifers: Quantitative Data
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Abstract

Needle leaves of Pinus longaeva can be accurately dated and remain alive on branches for 30 or more yrs, making this species ideal to study secondary growth in leaves. Field observations and regression analysis of needle age versus mean needle length both indicate primary (elongation) growth of needles is completed in the first year. Statistical analysis of cell counts for one- to 33-yr-old needles indicate production along the entire length of needles of 1.0-1.7 cell layers per year of secondary phloem, but no secondary xylem. Microscopic measurements and cell counts reveal that with advancing needle age phloem increases radially and transfusion tissue buckles, but the number of endodermal cells and xylem width do not change. Living phloem remains constant in amount (ca. 9 layers) with advancing needle age, indicating yearly replacement of old by new phloem. For comparison to P. longaeva, needle leaves were also studied for ten other conifer taxa with maximum needle longevities ranging from 3 to 19 yrs. In needles of each taxon no secondary xylem is produced, but secondary phloem production occurs throughout the post-elongation lifespan of the needles regardless of maximum needle longevity.

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