The Strasburger Cells — Equivalents of Compagnion Cells

The Strasburger cells in the phloem rays of pine and larch show most prominent structural and physiological differences from ordinary ray parenchyma cells. Besides their particular one‐sided sieve areas connecting them with the sieve cells, it is the absence of starch, the relative density in protoplast, and their premature death which makes these cells resemble true compagnion cells. Density in protoplast is primarily due to the missing of large vacuoles and the presence of numerous mitochondria while the ribosome population is not visibly increased. Strasburger cells in contact with mature sieve cells are distinguished by additional peculiarities, i.e. the highly elevated activity of mitochondrial enzymes and of acid phosphatases, the extraordinary prevalence of polysomes, the augmentation of nuclear surface and of the amount of decondensed chromatin, and by the increased nuclear RNA content, that all suggest an essentially increased metabolic activity of these cells. In DNA content, however, no significant difference is found from other ray cells. As these peculiar characteristics are not seen in Strasburger cells bordering immature sieve cells, they are taken to be not only of functional significance but to assure also the peculiar role Strasburger cells contiguous with mature sieve cells are playing in loading/unloading of sieve elements and/or in the sustaining of their metabolism.