Secretory structures of aromatic plants

To know more about how and where plants produce essential oil I bought the booklet:
'Secretory structures of aromatic and medical plants' 

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. 



Lavender oil gland 

In the Labiatae family, and thus all kinds of lavender, you come from a very different structure. On the epidermis of the plants are spherical glands with two cells connected to the epidermis cells.



 Figure 3: Glandular trichomes in Lamiaceae. o=oil drop, sc=subcuticular storage cavitiy, h=head cells (site of terpene biosynthesis), s= stem and b= basal cells. (Modified after Fahn, 1979a)

Stems and flowers of flowering plants are also littered with these glands.

Marjoram SEM photo



Like marjoram (Origanum Majorana) are photographed with a scanning electron microscope (SEM), and below a photo of rosemary.

Such structure also explains that if you touch a plant essential oil is released.
Even if plants like lavender is dryed when sqeeuzed the lavender oil kept in the glands will be released.

 cross section of a needle of a conifer



An even more complex structure are secretory ducts. Such channel-like structure found  for example in the conifers. Such ducts make a connection from the roots of the tree to the leaf, flower and fruit.

 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.

These channels  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.The oils  are biosynthesised within their leucoplasts and move via the endoplasmic reticulum into the cavity. The ducts are relatively close to the surface of the leaf. Not without reason. The smallest animals will take off  immediately after a bite out of a pine needle because of the bitter taste which is given by the resins. This is a protection for conifers.
Who wants something more to read about about plant anatomy: http://www.sbs.utexas.edu/mauseth/weblab