The genus Cinnamomum

Botany and Crop Improvement of Cinnamon and Cassia 25
mature mucilage cells in the leaves and shoot apices of C. verum and C. burmannii. In
all mucilage cells a suberised layer is formed in the outer cellulosic cell wall at a
very early stage in development. Oil cells are also known to have suberised cell walls.
Many features can distinguish these two types of secretory cells. A typical oil cell has
a three-layered cell wall – an outer cellulosic layer, a middle suberised layer and an
inner cellulosic layer. An oil drop is often attached to a protuberance of the wall, called
the cupule, and is enveloped by the plasmalemma (Maron and Fahn, 1979). Oil is
usually synthesised in the plastids (Cheniclet and Carde, 1985). Mucilage cells usually
lack a suberised layer, C. verum is an exception to this. Mucilage is produced in the
Golgi apparatus, from which vesicles filled with polysaccharides move towards the
plasmalemma and fuse with it. Mucilage accumulates between the plasmalemma and
cell wall (Trachtenbeg and Fahn, 1981). The two types of secretory cells can be
distinguished by the staining reaction with Sudan IV, Chrysoidin and Alcian Blue
(Bakker et al., 1992).
Development of oil and mucilage cells
Bakker et al. (1991) reported the development of oil and mucilage cells in the young
leaf and shoot apex of Cinnamomum burmannii, on which the following discussion is
based. Oil and mucilage cells occur in relatively large numbers in a zone underlying
epidermis, i.e. in the outer cortex of the shoot and in the mesophyll of developing
leaves. Three arbitrary developmental stages can be distinguished in both types of
idioblasts; the first two are the same for both oil and mucilage cells (Fig. 2.3).
Stage 1: The young idioblast, possessing a large central vacuole, is recognisable by
the absence of osmiophilic deposits in vacuoles (which occur in the vacuoles in the
adjoining cells in plenty). There is a parietal layer of cytoplasm that is slightly more
electron dense than that of the surrounding cells. The plastids are distinctly small with
reduced thylakoids.
Stage 2: At this stage a suberised cell wall is deposited in the cells. This layer consists
mostly of two to three discontinuous lamellae and sometimes up to six lamellae.
Subsequently, additional wall material gets deposited. The cytoplasm at this stage
contains many mitochondria, small plastids, and a slightly increased amount of
dictyosomes.
Stage 3 (Oil cells): Development during this stage differs in oil cells and mucilage
cells. In the oil forming idioblasts a distinct layer of inner-wall material gets deposited
against the suberised layer. Thickness of this inner wall layer increases significantly
as development progresses from 44 nm to 162 nm. Based on thickness of the inner
cell-wall layer and the composition of the cytoplasm, stage 3 can be subdivided into
three intergrading developmental stages – a, b and c. In 3a, the cell contains a
suberised cell wall and an inner wall layer. The large central vacuole disappears and
is replaced with a few smaller ones having wavy outlines. The cytoplasm is more
compact and contains a small oil cavity. In stage 3b, the inner wall thickness increases
and the oil cavity increases in size. A cup-shaped cupule that is attached to a thickened
part of the inner wall layer (cupule base) is formed. At first the oil cavity is
distinctly enclosed by plasmalemma, which later disintegrates. Plastids disintegrate,
paving the way to the formation of many small vacuoles, and they later fuse with the
oil cavity. In stage 3c, the cell reaches maturity, the suberised layer is about 36 10 nm
thick and appears as a translucent layer. The inner-wall layer (about 162 69 nm