Distribution and development of the non-articulated branched laticifers of Morus nigra L. (Moraceae)

The origin, development and arrangement of the branched, nonarticulated laticiferous system of Morus nigra L. (Moraceae) were studied in embryos, seedlings, and leaves, stems and roots of young and mature plants using light and scanning electron microscopy. There are two laticiferous systems present in the older plant, a primary and a secondary one. The primary laticifers arise from eight initials in the outer periphery of the future procambium near the cotyledonary node of the young heart-shaped embryo. They produce the first-formed laticiferous system of the plant. No additional primary laticifers are formed in other primary tissues. Secondary laticifers of the same type as the primary ones are produced by initials of the vascular cambium in the secondary phloem of stem and root. Their number is larger and variable. No fusions of laticiferous cells with one another or adjoining parenchyma cells were observed. The extremely elongated, thin-walled, branched laticiferous cells are devoid of pits. They exhibit a combination of intrusive and symplastic growth; penetrations into other cells were not observed. Laticifers are present in all vegetative tissues with the exception of epidermis, and primary and secondary xylem. The close developmental and distributional relationship between laticifers and phloem suggests a possible functional relationship.