This book, written by Katerina P. Svoboda and Tomas G. Svoboda, gives sharp microscopic images of the different structures in the plant responsible for producing oil. There are several oil-secreting structures:
The simplest structure consists of one single cell. This leaves of the magnolia has individually cells producing essential oils.
The system is simple: oil is sampled in the vacuole (in cell) equipped with a membrane. The cells are isolated and have an other internal cellular structure than the cells around it. Biosynsthese of all the constituents of the essential oil takes place in plastids (same type as the organelle chloroplasts) and in the cytoplasm of the oil-producing cell.
There are also plants such as Eucalyptus, who possess a cavity surrounded by a large number of oil producing cells within the plantstructure. These so-called excretory cavities are the place where the plant produced essential oil are being stored.
secretory cavity in citruspeel
Even citrus fruits like orange have such cavities lying in the epidermis of the peel. This also explains that when peeling an orange you smell citruslike odors.
Suchstructurealso explainsthat ifyoutouchaplantessentialoilis released. Evenifplants like lavender is dryed when sqeeuzedthelavenderoil kept intheglands will be released.
Anevenmore complexstructure are secretory ducts.Suchchannel-likestructurefound for example in the conifers.Such ducts makeaconnectionfromthe roots ofthetree totheleaf,flowerandfruit.
Magnification of ducts in wormwood. There are two ducts here, the one on the left is old enough to have developed a small lumen and epithelium; the one on the right has not yet reached the stage of having a lumen. The arrow points to an area where the cells might be starting to pull apart – the black material between the cells might be the first stages of the breakdown of the middle lamella (there are places in the upper part of this micrograph where there are similar amounts of dark material between cells
Transverse section of needle leaf of pine (Pinus). The needles of pines and many other conifers have resin canals with complex epithelia.
Thesechannels are composed of an epithelium which surrounds a central cavity. Some of these cells forming the wall of the cavity will change into secretory epithelial cells.Theoils are biosynthesised within their leucoplasts and move via the endoplasmic reticulum into the cavity. The ducts arerelatively closetothesurfaceof theleaf.Notwithoutreason.Thesmallestanimals will takeoffimmediatelyafter abiteout ofapineneedlebecauseofthebittertastewhich is given by theresins.Thisisaprotectionforconifers. Whowantssomething more to read aboutaboutplant anatomy: http://www.sbs.utexas.edu/mauseth/weblab