Electron Microscopy of the Development of Needles of Pinus nigra var. maritima

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Abstract
Pine needles were sampled at intervals throughout their first growing season and fixed for electron microscopy with potassium permanganate, glutaraldehyde/acrolein, and glutaraldehyde/formaldehyde. Mesophyll cells had electron-opaque tannin deposits and invaginated walls. Resin-canal cells contained characteristic plastids sheathed in endoplasmic reticulum; resin production was limited to the young needles. No distinct Casparian strip was visible in the endodermis. By varying degrees of wall thickening and vacuolation meristematic transfusion tissue developed into tracheids, parenchyma, or albuminous cells. The xylem had the usual structure but the phloem sieve cells had very thick fibrous walls. The cambium was inactive outside the fascicular sheath.

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