The Time Factor in the Differentiation of Secondary Xylem and Phloem in Pecan
American Journal of Botany 37(1): 15-24
ISSN/ISBN: 0002-9122 DOI: 10.1002/j.1537-2197.1950.tb08156.x
Freshly collected samples were cut on the sliding microtome and stained with chromotrop dissolved in 100% ethyl alcohol. The phloem stains a deep wine-red, the xylem weakly or not at all. The seasonal phloem increments are separated by a narrow zone of flattened, small-celled tissue containing undifferentiated cambium derivaties. The old phloem shows no breakdown and presents a normal appearance over several yrs. but is probably functional for only one season. Phloem and xylem differentiation are synchronized except in the beginning, when xylem formation far outstrips phloem development. The development of either tissue tapers off by midsummer. Phloem cells continue to form until the onset of cold weather catches the groups of phloem in a partially differentiated state. The storage tissue of the xylem is packed with starch at the end of the season; there is little food stored in the phloem where deposits are limited to the cells of the rays. Unfavorable environmental conditions may cause a reduction in both xylem and phloem increments but the size of the xylem increment is more sensitive to variation.