Graduate Thesis Or Dissertation
 

Anatomical changes in the secondary phloem of grand fir (Abies grandis (Dougl.) Lindl.), induced by the balsam woolly aphid (Adelges piceae Ratz.)

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  • The balsam woolly aphid (Adelges piceae Ratz.) feeds by inserting its stylets intra- and intercellularly into the cortex or outer phloem of the true firs. While feeding, the aphid injects into the bark an unknown stimulatory substance which affects the adjacent cortical parenchyma cells, the activity of the vascular cambium and the differentiation of the xylem. This study examines non-infested, infested, and aphid-aban doned trees of grand fir (Abies grandis [Dougl.] Lindl.) collected through the year by serial cross, radial, and tangential sections of the cortex, secondary phloem, cambium, and a portion of the xylem. The cortex of grand fir is composed mainly of parenchyma cells filled with resins, tannins, and sometimes crystals. Some of the parenchyma cells differentiate into astrosclereids. After infestation the cortex is characterized by hypertrophy of the nuclei and cortical parenchyma cells. As these cells enlarge into giant cells, there is an increase in density of the cytoplasm. Later a cork cambium is initiated around the pockets of giant cells. Reactivation and cessation of the cambium in infested, non-infested, and aphid-abandoned trees do not differ any more than the natural tree-to-tree variations that are due to location and exposure. The dormant cambium of infested trees maintains a wider radial file of cells in the cambial zone than do the cambiums of non-infested and aphid-abandoned trees. The cambium of infested trees generally produces more cells per year by increases in both periclinal and pseudotransverse divisions spread more or less evenly throughout the year. Pseudotransverse divisions in the non-infested and aphid-abandoned trees are generally limited to the latter half of the growing season. The sieve cells of the non-infested and aphid-abandoned trees are longer than those of the infested trees, but those of the infested trees are larger in radial width. The tangential widths do not vary. Rays are produced by anticlinal divisions of fusiform initials and by decline of fusiform initials. More declining tiers are noted in infested trees than in non-infested and aphid-abandoned trees. The fusiform initials decline to form one or more ray initials. In the latter case, the segmented initial produces two or more separate ray initials by unequal periclinal divisions and by maturation of some initials in the strand. The phloem rays usually accumulate a greater abundance of resins than do those of non-infested trees. The cambium of infested trees produces more tangential bands of phloem parenchyma cells and fiber sclereids than that of non-infested and aphid-abandoned trees. Traumatic resin ducts are initiated in the differentiating xylem, especially in heavily-infested trees. In the material examined, these were initiated in the spring, but they can occur at any time during the growing season. After prolonged presence of feeding aphids, phloem ray cells develop abnormalities such as dumbbell-shaped nuclei or a binucleate condition. Astrosclereids and resin cells were noted in all categories of trees studied. The astrosclereids were seen in all samples collected, whereas the resin cells appeared only in certain trees of the various categories.
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