Ultrastructural Aspects of Primary Phloem. Sieve Elements in Poinsettia (Euphorbia Pulcherrima. Euphorbiaceae)

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Certain developmental changes of sieve elements in the primary phloem of Euphorbia pulcherrima Willd. ex Klotsch (Poinsettia) were examined in the leaf blade, petiole and young stem. The sieve elements have simple sieve plates and, as seen in transection, each is associated with a single companion cell . Since, as expected, our fixed material showed a deposition of callose in sieve plates and in the sieve areas joining the sieve elements with the companion cells, the course of development and distribution of this callose was recorded. The sieve elements contain two types of proteinaceous inclusions, both cytoplasmic in origin. One is the typical form of P-protein composed initially of tubular units; later, the tubules become striated fibrils by stretching. The second inclusion is a nondispersing protein body, the first distinct feature identifying the differentiating sieve elements as such. It persists during the differentiation of the cell and is present in mature sieve elements. Aggregated endoplasmic reticulum is rather sparse in sieve elements, but the typical stacked form is occasionally observed in mature cells.